C.S.
We have a 5 year old girl about 39 inches and 40 lbs and a 4 year old boy about 40 inches and 43 lbs. They are both in Cosco brand child seats that use the seat belt through the back of the child seat. They are 5 point harnesses that work great. They have lots of head room and they know how to unbuckle and buckle themselves in. They are not big enough yet for only the booster. Hope this helps.
C. R. in Hillsborough