Suspenders will hold up pants that have no belt loops, but they have to go over the shoulders....
Kid's Suspenders & Mitten Clips that site has been tempting me many times in the last year or two.
I was going to recommend dapper snappers for the pants with loops too. I usually buy overall sets (or dresses for Rebecca) for my kids. I need to get some dapper snappers for them though, because we have a lot of hand-me-downs with loops.