i dont have kids yet but plan to. and my home uniform is undies and half the time a bathrobe over. that's never going to change. the only times i wear more is if i have in-laws visiting [they stay for months at a time xD] - or if it's crazy cold.
i think it's important to be confident and teach kids not to fear their own bodies, as I was taught when I was younger. BUT being in undies should also be considered non-sexual. i would never be explicitly exposed with my/or any child [but nudism is totes fine if it suits your family, by explicit i only mean aroused or in a sexual nature]. but it's totally normal to hang around the house in skivvies, no matter what that looks like. my nephew runs around in his diaper at my sister-in-law's house and his dad wears just boxers. that's totes no biggie.
for me, wearing thongs 15+ years now, they're just my undies of choice. my husband is not phased anymore by it and he lives in full pijamas [much to my dismay] but that's fine too!
normalizing is de-stigmatizing.
that said, if they were interested in thongs themselves, i probably would not buy them thongs until their teens, maybe older, because i'd want them to have a job and pay for their own stuff lol. Want a videogame? Wanna go to the movies with friends? want comfier undies? get a job.