I only had one or two enemas as a kid, and I think they were lying on my stomach, but not sure. I hated them then and found them really embarrassing, so I think I tried to forget as much about them as I could. I had a chronic illness and had to undergo a lot of invasive tests. It wasn't a fun time in my life and there are parts of it I have tried to forget.
Once I got into enemas as an adult, I prefer knee/chest or hands and knees. Completely exposes the person getting the enema and shows off my favorite view of the butt as well. I've never tried getting or giving an over the lap enema, but love that idea too.