Yeah, this is very important...
Especially when it comes to driving on grass.
I'd recommend starting at 1/8th scale for proper grass duties, but then wouldn't recommend them for young children really, just because then can be a bit of a handful when you lose control and, because they are bigger and heavier, can cause more damage to what ever (or who ever) they hit. Having said that, had I had the money at the time, I'd have had no qualms at all about letting my lads, at around 6~7, have a 1/8th buggy...
I've driven my 1/10th scale rally car on grass, but it has to be really short to be able to cope with it.
My 1/8th buggy seems to manage quite well in grass up to four inches high, at least, and only gets stuck on some of the larger tufts, or launches into the air off them
I hope you find something suitable Andy