We give the kids Bonine. It lasts all day and never makes them drowsy.
As far as the belts on the roller coasters, I went on a couple of them at Kings Island a couple years ago when I was at about 235 (and I'm only 5'2") with a waist of about 48", hips about 53",(can't believe I was that big!) and it was snug, but I fit. Check out the websites. Most of them have size recommendation and cautions for all the rides.
Most of all-enjoy your visits! I loooooove roller coasters and may get to go back to Kings Island this summer