I would personally never sit under a horse, like others have said, this is rather dangerous. However, I think a quick 'from one side to the other' thing is alright. I've done it with my horse, and he couldn't have cared less.
When I was talking with a trainer / breeder friend of mine a while back, she told me that she did anything she could imagine a kid doing. ( as she primarily marketed her horses to beginners / kids). If you think about it, kids are a perfect height to just walk under, so it makes sense to desensitize a horse to that.
" If the entire world was one big machine, I couldn't be an extra part. I had to be here for some reason." ~ Hugo Cabret