Four Ways a 15-year-old vampire brat could obtain blood.
Method One of Three: Make a blood bank employee give them blood.
Archerspk suggested that a 15-year-old vampire could turn an adult who worked in a blood bank into a vampire and then have them provide blood for the kid vampire. But Starfish Prime wondered why an adult vampire would bother feeding the kid vampire nistead of disposing of them.
Could a 15 year old vampire physically handle an adult vampire if the younger vampire couldn't control the older one through magical methods? That depends on the physical conditions of the 15 year old vampire and the adult vampire.
One thing which I have learned is that there is a great range in possible growth rates for boys and girls, and thus a great range in possible sizes and strengths of various boys and girls at any particular age, including age 15, though of course the majority of children and fictional child vampires would be close to the normal size for their ages.
Once I used to buy lunch at a diner where one of the workers was a boy about 5 feet tall who looked like an exceptionally cute child actor. One day at dinner in a restaurant in the same little town that boy was part of the waiting staff and one of my companions asked if he was old enough to work there. He said he was 16, (probably the minimum leagal age to work there) and she said she didn't believe him, that he looked 12.
Another time I met another boy, whose age I don't know, who was exceptionally cute, but this boy towered over my five feet eleven inches (1.5748 meters) self. I think he should have been about six and a half feet (1.9812 meters) tall, and judging by the work he did helping us pack up, I guess he wouldn't have been less strong at age 15 than most men.
But those are not the extreme possible examples.
My historical research about a bugler boy in the US Civil War showed he was allegedly 14 in 1860 and five feet eight inches tall, 19 in 1865 and five feet three inches tall, and 22 in 1869 and five feet Eight inches tall, having grown five inches since the age of 19. And though the official records often said boy soldiers where older than their actual ages, a birth date in 1845 or 1856 is consistent with his age in later census records.
I note that the organization Little People of America had more than 6,800 members in 2010, and membership is restricted to:
Membership in LPA is limited to people 4' 10" and under, or those with a diagnosed form of dwarfism, their families, or those who "demonstrate a well-founded interest in issues relating to dwarfism".
So the USA population may contain thousands of adults under four feet ten inches tall, who weren't any bigger when they were 15-year-old boys and girls.
Of course some people at the upper size limit of of the Little People of America could still be very strong. I once saw a man who was about five feet tall who was built like a gorilla and looked like he had tremendous upper body strength.
These days it is common for parents to give synthethic human growth hormone to their kids if they are undersized. Thus 15-year-olds who are small for their age may be rarer now than in the previous century.
I note that the six shortest women over the age of eighteen in history ranged from 61 centimeters (24 inches) to 71 centimeters (28 inches) tall, and the eight shortest men in history ranged from 54.6 centimeters (21.5 inches) to 74 centimeters (29 inches) tall. I think that all of them should have been much weaker than any typical adult when they were 15 years old.
Angus MackAskill (1825-1863) reached seven feet four inches (2.24 meters) at the age of 18 and soon reached his adult height of seven feet ten inches (2.39 meters). His early adult weight was 580 pounds (260 kilograms). I think that at the age of 15 he would have been much stronger than most adults.
Louis Cyr (1863-1912) was one of the strongest men in history, despite being only 1.74 meters (five feet eight and one half inches ) tall and weighing "only" 124-154 kilograms (280-340 pounds). At the age of 17 he won a weight lifting contest lifting a 480 pound (220 kilograms), at the age of 18 he lifted a full grown horse. At the age of 15 he would have been much stronger than most adults.
Robert Pershing Wadlow (1918-1940) reached a height of eight feet eleven point one inches (2.72 meters) and a weight of 439 pounds (199 kilograms). Unlike most of the tallest men in history:
He possessed great physical strength until the last few days of his life.
He reached a height of seven feet ten inches (2.39 meters) and weight of 354 pounds (161 kilograms) at the age of 15 and should have been strong enough to handle most adults.
There are some very strong female wightlifters and bodybuilders, and I think that many of them would have been stronger at the age of 15 than most adult men.
Anyway, I guess that about 50 percent of 15-year-olds are at least as strong as adults of their own gender and ethnic group, and about 50 percent of 15-year-olds are no stronger or even weaker than adults of their own gender and ethnic group. If the bloodthirsty kid is a 15-year-old American boy, he might be stronger than the average adult American woman.
Method Two: Work in a Blood Bank?
The assumption that a 15-year-old vampire brat couldn't work in a blood bank may be unfounded. I once entered an ice cream parlor in Cape May, New Jersey, where three boys about twelve years old were behind the counter. They looked like triplets, and I guess their parents owned or operated the store, because I think that most states in the USA have exceptions to their child labor laws allowing children to work at their parent's farms or businesses at much younger ages than they could work for unrelated employers.
Or maybe those kids were working illegally despite being in view of every customer who walked into the store.
So if any blood banks are private small scale mom and pop businesses a kid 15 years old or younger could legally work there. Or maybe the kid works illegally in a blood bank, more or less hidden in the office away from visitors, cleaning up and doing paperwork - if that paperwork includes keeping track of incoming and outgoing blood it would be great for the vampire brat. And maybe the kid sometimes has to dispose of blood that spoils and collects some of it for their own use.
Method Three: Work in a Farm or Slaughterhouse.
Or maybe the kid works on their parents' farm raising and slaughtering livestock, and maybe they are in charge of collecting and selling or disposing of blood from the slaughtered livestock, and has their own methods of handling some of that blood. Of course that is if animal blood will satisfy the vampire brat. And if the story is set in a developing country with lax child labor laws, the vampire kid could work in a huge slaughterhouse where nobody tracks the amount of blood spilled.
Method Four: Claim to Need Blood for Medicinal Reasons.
Such a claim would probably be the literal truth for a more scientifically plausible vampire brat. So if the vampire is of the less fantastic type and medical tests would show they are alive and not undead, maybe those medical tests would reveal that they need blood, and they could legitimately get transfusions of blood as Willk suggested.
Back about 1960, I was told that a woman friend of my parents had a medical condition which required her to drink a glass of ox blood every day. If that was not a tall tale, there may really be such a medical condition, which might possibly still be treated by prescribing animal blood. And if animal blood is good enough for the vampire brat, they might claim to have that condition.