The strength of someone's mana is determined by how much experience they have
My idea here is that an individual's mana is shaded and molded by their experience and learning. A person's mana getting stronger as they age isn't a result of it increasing its raw power, but of the person's understanding, wisdom, knowledge, and personality growing and becoming more defined. This allows their mana to take action more efficiently and more effectively, but it makes it more difficult for someone else to direct.
Mana only likes to be controlled by whoever made it
Normally, mana only acts perfectly in accordance with its creator's will, becoming perfectly malleable to whoever it came from. However, this malleability only persists as long as that person's will exists to guide it. Should that person's mana be taken from them somehow, another person would have a much more difficult time actually directing it.
Mana removed from its owner, by whatever means, has a tendency to act out. This is because mana itself almost seems to have a will. In particular, it seems to have a desire to only be directed by whoever made it - not that it's actually conscious or intelligent - and it will fight against anyone else who tries to control it.
Age makes mana more complex
As a person gains experience, their mana becomes more complex. This is a result of the mana generated incorporating an individual's experience and personality into itself - or more accurately, being generated from a more complex personality.
The more complex this mana is, the more devious it is likely to be in its attempts to avoid being used. Remember that mana's complexity comes from its owner's own experience. Someone who is older has had more opportunity to gain experience, and thus their mana will generally be more complex than that of someone younger.
Children, on the other hand, have virtually no experience, so their mana is incredibly simple. This limits what it is able to do, but this also makes it easier to control. In effect, it is a lower quality, but far more malleable, supply of mana.
This makes children's mana easier to use, and thus more desirable
Children's mana is technically lower quality in that it is simpler and cannot perform a task as efficiently or effectively as more complex mana. This same trait makes it easier to control and direct, as it is less capable of resisting.
Now, when someone needs a ritual done, especially a complex ritual with many parts that could easily be disrupted, they want mana that they can reliably control, and therefore, they want mana harvested from children.