Kids, children or minors
Kids
Informal: chats, WhatsApp, talk with friends.
The kids played cricket till it got dark.
Children
Neutral: class, essays, most writing.
The library has a special corner for children.
Minors
Formal or legal: people under eighteen.
Minors must have a parent's consent to join.
Remember: WhatsApp: kids. School essay: children. Rules, forms and notices: minors.