I think its because kids take people at face value (with genderqueer/androgynous they may take a look at bigger body language signals to determine). They are new to the gendered world we live in, and do not know the main physical "tells" that adults do. They take you as you present yourself. Be great if everyone could do that. Sadly, these children will likely be brainwashed by gendernormative society as they grow up.
On that note, I've also had children see me as male. One little girl vehemantly argued with her grandmother about it in front of me...I didn't know what to say.