First question first, you just put the dob on the ground and use it there. You will need a chair/stool for when the eyepiece is low but the eyepiece will always be easy to get to.
Second question, I would not go for a GEM for a kid. It has to be aligned, the eyepiece can end up in some really awful positions as the scope tracks objects, they are much more complicated than a simple dob mount.
For the same money have a look at the
6" dob here. Simpler to use and MUCH more light gathering ability.
Cheers