I use them with my 3-1/2 yo DD at night. I put in two inserts, and we don't have leaks. I also love the inserts to stuff in her little brother's nighttime diapers (Simplex OS and BG 4.0s)
My DD is only 28 lbs, so I can't tell you how large the trainers actually fit. However, we did have a Grovia trainer that seemed too tight on her, and she was not able to pull it up or down.