You can describe an adult or a young person as childish when they act in an immature way, as if they were still a young child.
1. She hoped that he wouldn’t settle into one of his gloomy childish moods and spoil the day.
2. I thought her nice but rather childish.
You can describe an adult or young person as childlike if they shows some of the attractive qualities that children have, especially simplicity and sincerity.
3. …his childlike charm.
4. She was a dear old thing, but childlike and eccentric.
Instead of saying that an adult or young person is childish, you can say that they behave childishly.
5. Even the competent adult may become childishly helpless or demanding when ill.
Note that neither ‘childish‘, ‘childishly‘, nor ‘childlike‘ is normally used to describe ordinary things to do with children. Instead you use children’s or for children.
6. Good children’s bookshops are few.
7. There are special shampoos for children.