A ".40 size glow trainer" refers to the engine size. A .40 (or 40) has a .40cuin displacement. It turns a 10"=11" prop in the 10k-18k rpm range. The plane will weigh 5-7lbs, have a wingspan around 50-70", and travel around 50-70mph in a straight line (top speed, they can fly slower).

It's the most recommended way to learn because the larger size means the plane is less bothered by small gusts of wind and such, and the plane is easy to see for most people. However, you do need a dedicated flying site to fly one of these, getting hit by it can be fatal, and getting a finger in the propeller can cause serious injury.

I wouldn't give a 10yr old a .40 trainer by himself. If you are also learning to fly, and you do it together as a father-son thing, then you're fine. I have a student who I just signed off as "soloed" who is only 11 (I think). His father doesn't fly, but is always there and always helps with engine starting. I didn't let the kid start the engine for a while, until I had time to walk him slowly though all the safety stuff with his father, so they both know what they are doing. It didn't hurt the lesson that another guy went in to the prop the other day, and really woke up my students that I'm not kidding about how much of a mess they can make.

So, respect the engine and prop, never rush, always be careful, and you'll be fine. Go at it with out knowing what you are doing or in a sloppy way, and you can get hurt.

Oh, I was 14 when I started to fly. My Dad would drop me at the field, but I had some good instructors, and I always respected that engine. Personally, I've never had an engine draw blood on me.