My 4 year old wears size 6 and my small 6 year old wears size 7 and they are both still very snug in the arms, but otherwise sizing up does the trick. Also both of my boys are skinny, for a thicker child I would go up another size or maybe just skip this style as they won't be comfortable. The short sleeve are not as bad as they don't have the sleeves so sizing up 1 size works for my boys in that style.