Are there some common guidelines about how big or old kids should be before
going for a glider ride? My concerns are around the safety harnesses being
intended for adults...

How do you ensure the harness is safe and is not going to slip around or off
a kid?

Are there safe ways to use a booster seat to enable a small one to be up
high enough to see out, without a possibility of it becoming a loose object
in the cockpit?

Anyone have experience with this subject?