I have a 4 year old & a 2 year old who are both about the same size. My 4 year old is about 29lbs and my 2 year old is about 31lbs. (I know she’s huge lol but she’s also tall). The one size fits both of them perfectly. The strap adjusts in the back to go as tight or as loose as you need. The arm parts are easy on and off and the entire thing stays put. Doesn’t move up on their necks or anything! I’m glad I purchased these ones. & the designs are cute