My kids are ages 8 and 9. Is there a different cost for them for the Navigo Week Pass?

Navigo Decouverte passes do not have children fares.  The price for Navigo passes are the same regardless of age.  Only local resident children have access to a school version of Navigo which comes at a yearly discount.
 
Navigo will still be the cheapest option for children when compared to Paris Visite passes.