Navigo Découverte or carnets?

It looks like you will need carnets for Thursday to Sunday Midnight, then Navigo pass for the rest of the trip. One big advantage of the Navigo is the inclusion of RER to CDG in the fare (not included in the carnet). Don't worry about spending a few extra Euros, time to relax and have another cafe au lait.

I would use carnets from arrival to Sunday midnite, then Navigo weekly pass for the rest of the visit.

This is a nickel dime decision; does it really matter if you spend a handful of Euros more or less in a trip this costly? The ND is so convenient that I would certainly use it for the second week; it will easily pay for itself if you use it for the trip back to the airport. The first week, I'd probably just get a single carnet and not worry about a child's carnet since you won't be using it much. The ND is so cheap that it is hard to go wrong though. But if you don't use it to return to the airport, then it would probably be cheaper to just get a carnet of adult and a carnet of child tickets and do a lot of walking and then buy another carnet if needed. With your schedule the carnets will be cheaper especially if you use a cab to the airport. Child carnet are for kids 4-9.

Unless you are going to save significant money with the carnet, go for the Navigo. It's so easy and there's no keeping up with which tickets are new and which are used. Plus, if you just want to hop a bus to go a few short stops, you're not blowing through your tickets.

Sounds to me like the Navigo, which I loved having on our last trip, will likely cost you more.

Thank you for your replies. It is certainly a price vs. convenience dilemma. I loved the ease of having the NDs last time. It's strange that there is no child rate on the ND. Have not been able to figure that out.
I will go with the NDs.