If you love your grandaughter at all, you won't let her actually mountain bike on a cheapie bike. They're suitable (barely) for slow neighborhood crawls, but off-road they're accidents waiting to happen, and mountain bike accidents can be painful and crippling.
Oh no! Neither one of us ride in the mountains or on rough trails. If we ride it's on a paved country road or at her school. She's only 10 and we don't allow her to ride without me and I don't do trails. :-) We have to ride them because they do better on country roads where you encounter a lot of gravel, etc. One of these days when she learns to treat her things better I'll get her a better bike but right now we have to keep after her about putting it up, etc. If we didn't make her bring it in it would be on it's side in the yard. Granny does not do bike trails! :-)