My 5 and 3 year olds are the same weight and height and both have red hair so we get asked that alot too.
Recently we brought our 8 yo to the Dr and we took the 3 and 5 yo with us. As we were leaving, DH had my 5 yo's hand, I had my 3yo's and our 8 yo was walking along side. Some woman smiled at us and told us we had our hands full. DH and I just grinned at each other and kept walking. If she only knew :)