H.B.
I have a really small 5 year old boy and I buy the pants that are adjustable but I've also used mitten clips (the ones that you use to attach the mittens to the jacket. I found them at Target. A 2-pack is 1.99 I think) I would attach one end on his belt loop and attach the other to the other loop. Usually skipping one loop. They are elastic and he can still get his pants down to use the restroom with out help. Good luck
H.