My son is currently between sizes. In order for the waist to fit, I have to buy 3T pants, but he has 2T legs. To keep his pants from looking like the above picture, I did this:
On the inside of each pant leg I sewed on a series of snaps. The bottom snap is the opposite of the two top snaps, so that I can snap the pants up at the highest right now, snap the lower snap later on, and then not snap at all when the pants actually fit. Because the pants snap, I can reuse these pants for any future kids I might have without having to re-hem the pants.
This pair of pants is the first pair of pants that I tried the snaps on. I've found that the snaps work best by placing a row of snaps at each seam, and then a single set of snaps in the middle front and the middle back. I used mid-sized snaps on the pants so that hopefully the unused snaps won't be uncomfortable later on.