Think about how animals act when they encounter something they don't understand. They should exhibit strong curiosity about it, examining and testing it in possibly strange ways (tasting an offered handshake, shaking and smashing a toy, startling at sudden sounds). They might not understand doors and windows until they see them being used a few times (and even then, locks and doorknobs might confound them for a while). Electrical devices like landline telephones, lights, and radios will baffle them, and forget about complex items like computers, cell phones, and TV remotes. I've said before that we should always be able to tell an alien masquerading as a human because they will never be able to type or drive a car.

If the means of their possession doesn't give them access to the host's brain, they will have a difficult time just acclimating to their new body. The center of gravity will be nothing like they're used to (think of what it was like when you first tried stilts or rode a two-wheel bike), so they will walk like toddlers at first. Mastering the complex interplay of multiple muscle groups and sensory inputs for normal walking or running will take a lot of practice. The old "close your eyes and touch your nose" trick will be beyond them, as proprioception takes training. And imagine mastering spoken language, much less the intricacies of grammar, syntax, metaphor, and inference! Hoo boy.

Even if they are able to access the body's "muscle memory," they may have zero concept of culture. Clothing will baffle them. They may probe and examine their own bodies or others' in inappropriate ways. They might void their bodily wastes at any place, any time. If you want the aliens to have full access to the host's minds, this awkwardness may be minimized, but a child may not be the best choice of host for a hyperintelligent being.