Quick sizing question for a toddler Sewing Machines Parts and Sales in Los Angeles

I’m working on a project atm to use up some extra satin I have and thinking of making a toddlers dress for a 3 year old. I’ve got two boys at home who are same age but no girls( dress is for their cousin) I want to surprise her with the dress so I don’t want to ask her parents for her measurements iam wondering if it’s safe to size the dress to my boys t3 cloths for her. Do toddler sizes differ by gender? Im thinking sleeveless and size the bodice to a boys shirt then guesstimate on the skirt length based on my boys. She’s roughly their size if more dainty.