M.Y.
My little boy who's a little over 2 years old is also skinny in the waist, but with long legs. Look for adjustable waist bands and I've had good luck with Old Navy and sometimes Gap. Target also has some..I think it's the Osh Kosh brand. When I see them on sale, I snag a few at a time because they're hard to find and pricey (especially Gap). Good luck.