RE: RC for Kids
I am not sure about Rustler but I would think Stampede would be better on grass due to higher clearance and bigger tires. But in terms handling, Rustler is way better and it does not flip easily. (From a kid's perspective, this is a good thing). I would recommend a Traxxas Stampede since it's waterproof, higher clearance. As far as the motor/esc is concerned, just buy a smaller pinion gear to keep the load down and everything is all good from there.
as for 1/16th scale, they are not very good on grass and they are not much cheaper sometimes even more expensive. 1/16 Revo 4WD brushed cost more than a 1/10 Stampede 2WD.