Adult characters are all the things they've encountered over time. But kids haven't accumulated all the life experience, all the regrets. They tend to be more in the moment, more willing to play, to be joyful.
I know people who prepare their roles in such a way that they technically look ahead and memorize their gestures, and then they stick to it. Those that are technically proficient enough can make it seem natural, but they do that and don't really take in what other people are doing.
I've been lucky to learn by playing all kinds of roles and watching all kinds of really good cinematographers, actors, and directors for many years before people were even aware of me in terms of audience.
If there's one thing I've learned from traveling, it's that it is definitely more important how you are than where you are. You can say, 'Oh, I hate X city, I hate that country, or I prefer this city,' but it's a little bit up to you to find some kind of happiness.