There is no disbelieving child even if his parents were disbelievers. Children are born on the distinct (Fitra) which has dasics of faith. No one will be punished by a Allah, The Glorious, before knowing and insisting on denying the truth after knowing the truth. Children who die in their childhood will be given on the Day of resurrection full option to know the truth then decide. If they accept the truth, they will be in the Paradise.
Wassalam.