This might sound counter intuitive, but bike parks are good for this sort of thing.
Most have easy green/blue runs that are ideal for kids and being all down hill they will probably love it. Most good parks also have loads of other things to do when not riding as well so it doesnt get boring for them. The fact that everything comes back to the same place means that adults can alternate runs on their own or one can go with the kids while the other rides solo. There is usually quite a few kids around as well for them to play with.
Valnord is excellent for kids, excellent blues and greens and loads of peripheral stuff like rope slides and go karts. Deux Alpes is similar but has even more kid friendly trail options, and loads of english kids of similar age at the campsite in Venosc.
I have been taking my lad to bike parks since he was 5, he always has a great time, especially as pedalling was optional 🙂
Just stay away from the busy resorts like the PdS as they would be potential carnage.