Talking to my non-binary child about underwear

• 4yo nb:
  why are my panties girl panties?
• Me:
  they're not girl panties, they're YOUR panties. If you're a boy then they're boy panties.
• 4yo:
  in the store though were they girl panties?
• Me:
  the store was selling them as girl panties but what do they know? Some girls have vulvas and some boys have penises so they think all girls like certain panties and all boys like others. The store isn't the boss of your underwear.
• 6yo daughter:
  and some boys have vulvas and some girls have penises!
• 4yo nb:
  And everyone has a BUTT!!!
• Me:
  exactly! Boys and girls aren't that different. So it's silly to say that all panties like this are for all girls or for girls only. Panties are just panties, they're for anyone who likes them. You like them, they're yours, you're a boy, so they're boy panties.