1. every adult is independent when applying for N400.
2. kids under 18 CANNOT apply for N400. However, kid under 18 living with you under your custody (or you are still married to the other biological/legally adopted parent of the kid) and kid is an LPR, then when you get your US citizenship, the kid gets his/her USC automatically. "Automatically" means no application or USCIS examination is needed.
2. kids under 18 CANNOT apply for N400. However, kid under 18 living with you under your custody (or you are still married to the other biological/legally adopted parent of the kid) and kid is an LPR, then when you get your US citizenship, the kid gets his/her USC automatically. "Automatically" means no application or USCIS examination is needed.