This is the OP just coming back to thank everyone for the suggestions. We ordered the playware pants from Tea Collection and they have been great--they don't fall down, have no fasteners, have pockets, and have a reinforced knee. Thanks!

My kid is also small and thin. Old Navy works best. Some Baby Gap. Also Zara Kids run narrow.

For us, Gymboree and most Carter's are cut wide.

Another vote for Primary for slim toddlers! Although I'm finding they're running out of colours these days - the word has clearly gotten out...

I 2nd H&M. The pants are quite skinny and are pull-up. Also, very affordable.

I've found Nike pants to have smaller waists than Under Armour and some other brands. The Gap knit pants have always been our go-to. When they're still a little big we just fold over the waist one time and they stay up.

My DS is super skinny as well, and we've done okay with the Gymboree pants that have elastic waists - so the whole waistband is elastic, not the kind with the elastic insert. Those inserts do nothing for him.

Also, with warm weather coming, you can buy shorts in smaller sizes. The shorts are very forgiving in length