I think child fans can be added to the demographics in the game since there are some idols that are 12 (or under if you make your own) and here’s how you can get them!
1. Performing at kid’s concerts( casual or hardcore)
2. Promoting products for kids such as toys( can either be male or female and casual)
3. Modeling for children’s clothes(casual but mostly female)
4. Children’s music can be a new song genre(not talking about nursery rhymes!) But you can make song covers from anime for kids such as Yume wo kanaete Doraemon, theme songs from the Pretty Cure franchise and more
5. Have them be featured in anime or manga for the kiddos
6. In the cafe, you can serve food for kids or a kids menu
And that is all have!
Fun fact: IRL some idols have kids for fans, for example, During Prizmmy’s final concert one of their pictures feature them with there fans (with most being children and teens)