if you're in a less diverse society or a more classist society. (Having never been to Argentina, I can only go by your post.) Perhaps address it as issues arise, as a teaching moment, even if after the fact? Something simple like, "Everyone is created equal & that's important for you to know, but there are people who don't agree." Or something like that. Whatever fits the situation you're addressing, the viewpoints that you want to teach them, & their age.
I do think simpler is better when they're young. So far race has not been an issue raised by ds #1, but we live in a melting pot area & they have friends of different races. So far ds #1 hasn't remarked about anyone's differences.
We do have several same-sex couples we're friends with who have kids the same age, & the kids have also grown up with a girl whose mom is a single mom by choice. At this point, ds #1 will just ask how come so & so has 2 mommies or say he wants 2 mommies. Last night he told me he wanted 2 mommies & 2 daddies. We just keep it simple & say some kids have 2 mommies, some have 2 daddies, some have a mommy & a daddy, some have only a mommy, & some have only a daddy. Just factual & without judgment, which is in alliance with our feelings & beliefs. Deeper discussions will come some day, but for now, that's enough at 4.