Would strongly recommend les deux Alpes
Les2alpes Summer Mountain Biking
If the kids can manage laggan red they would be able to manage the greens and probabally most of the blues.
It’s a big hill so you would want to take your time but the whole place is very user friendly, very well marked etc + the town is a great place as there is a summer ski scene there meaning most shops restaurants etc are all open, + golf, swimming pool, ice rink, paint ball, archery and loads of other stuff going on as well, unlike a lot of the resorts which are a ghost town in the summer.
+ alpe D’huez is only 1 hour away if you wanted to check it out although in my experience the green trails there are very limited and tend to have good riders riding them at ‘race pace’ as they are an easy way to get back to town from the upper mountain.