A Secretary bird or a Harpy eagle.
The secretary bird sounds like an ideal form. Even though they are birds, they spend most of their time walking on land, despite being good fliers (so they can climb no problem) are 4 feet (1,2 meters) tall (not too tall meaning they can likely go into spaces a toddler can) and have a kick worth around 5 times their weight. They're also known as "the devil's horse" . See the following:
"Secretary birds can run so fast that they've earned the nickname “devil's horse.” There is no concrete number on max speed, but they do a lot of walking and they can travel up to 18 miles (30 kilometers) per day!"
So we have a bird with good endurance, supposedly good speed and with little problems regarding climbing (they nest on trees and fly well, so shouldn't be much of a problem).
But maybe you might also be interested in a form that has even greater eyesight and hearing, meaning it can track intruders better, while still being able to navigate in closed spaces. If that's what you want, look no further than the harpy eagle.
If the picture doesn't tell enough, this bird is not only big, it's strong. Harpy eagles are considered the strongest eagles by many, having talons the size of a grizzly bear's claws, which it uses to hunt and dismember the monkeys it feeds on. In addition, it's native to rainforest biome, with its relatively short wings (1,7 to 2,2 meters from tip to tip) having specialized to fly fast through the dense vegetation. It can reach speeds of up to 50 miles (80 kilometers) per hour.
"Its short, broad wings help the harpy fly almost straight up, too, so it can attack prey from below as well as above. And the harpy eagle can turn its head upside down to get a better look at its potential meal."
This eagle also posses great eyesight, with better color vision than ours, along with a face disk that grants it good hearing. It's sheer size and strength also make it able to knock down humans with a well placed attack to the head.
"It has excellent vision and can see something less than 2 centimeters (1 inch) in size from almost 200 meters (220 yards) away." http://justfunfacts.com/interesting-facts-about-harpy-eagles/
So we have a powerful raptor with strong claws, wings adapted for flying in relatively confined spaces, good speed and supposedly good endurance, along with the ability to subdue a potential invader without needing to change form.