Definitely, NP is used by children in casual conversations with their friends or with family. It sounds quite friendly when they respond with NP to being thanked or asked for help. Since NP stands for No Problem,
it teaches children to be nice and considerate during their interactions. Children must know the appropriate and inappropriate places to use NP though. In more formal situations, like in school or with adults they do not know so well, saying You're welcome
would be preferable. But NP is surely a fun and easy expression for children to use during informal talks.
Know more: https://radarro.com/np-meaning/