Hi Kelly, I have done two FatTire bike tours and I thonk your son is too young. The youngest on our ?Versailles tour was 12 and he did ok, I think he had fun , but his parents were a bit stressed with the driving through the city part, and you do have to carry your bike up a flight of steps.
I have been to Chantilly , last year, and I loved it.( only about 1/2 hr on train) I found it such a nice surprise. I have been to Versailles many times now, and I love it, and I am going again, LOL, BUT, I would still return to Chantilly. It is smaller, more intimate, and WAY LESS CROWDED! But, I am not sure if an 8 yr old would enjoy it as much, I mean there is not alot of furniture inside, mostly art , as the palace contains a museum, a collection second only to the Lourves.
BUT, the grounds are lovely, and you can wander over to an outdoor play area and a outdoor cafe that serves many desserts featuring of course,, Chantilly Cream( sweetned whipped cream) .
Now, if you son likes horses, you cannot go wrong, as right across from street from the Chateau is a very good Musem of Horses, and the Royal Stables. I enjoyed it and I don't particularily like horses, LOL
Anyways you can google Chantilly Chateau, they have a nice website, in english if you click on icon.