Children and adults pay the same price for a passe Navigo Decouverte.
Your 12-year-old is considered to be an adult. Your 8-year-old is a child. The fare for a single adult ticket from CDG to central Paris is 8.50 EUR. The fare for a child ticket is 5.95 EUR. The ticket provides unlimited transfers until you exit the system.
Buying ten-ticket carnets as needed for your younger child will probably be a better deal than a passe Navigo Decouverte. The tarif reduit (reduced price) for a carnet is 5.80 EUR. (The adult price is 11.60 EUR.) A single t+ ticket is good anywhere the metro goes (some lines end in zone 3) and on the RER in zone 1. Each ticket provides bus transfers for up to 90 minutes after boarding the first bus.
On weekends both of your children would be able to travel on one-day Ticket Jeunes (for persons under 26). The zone 1-3 pass costs 3.30 EUR/person/day.