I don’t do that kind of riding but unintentionally ended up there one evening last summer.
They are at the top of the hill in the woods behind the massive (Hemlock) stone. As far as I recall, relatively small area but quite popular.
Just keep following the tracks up into the trees. If you drive there, there’s a car park the other side of the road at Bramcote Hills, which also has a play area if you have a little one, but no toilets. Coffee and/or ice cream vans sometimes in the car park.
It’s a slog up the hill to the trails, might not be great with a balance bike, and might not be a lot of space for them to play if it’s busy.