Then from Ballater, you could take the B976 to Crathie (which although not a track/forest trail is probably the best option).
From there though, it looks more tricky depending on how much Queenie is cycle-friendly, as all the tracks up to the Invercauld Bridge on the south side of the Dee are on her patch. From Invercauld Bridge, just pop over the A93 and you can ride past the Invercauld House and then back down to Braemar.
I think that would be a pretty long ride for kids that small though, unless they are asleep for most of it! 🙂
The Deeside Way is ideal, and riding that from Aboyne out to Ballater is great – really nice track, good scenery and some good stops along the way.