Was a drama teacher; had a very memorable lesson with a female batman (not batgirl) and a male Elsa... both aged about 5. Not one kid batted an eyelid, only really anxious people were the parents who presumed kids might hold the same sort of prejudices as adults around gender. Generally speaking they don't - and any lingering prejudices are easily swayed into acceptance by a lack of reaction from adults around them - on that occasion it was me. Went home, considered my own prejudices and realised the vast majority of my lessons most kids wanted to be a dog. That's the dream role for most five year olds. :o)