tbh, it would be really interesting to know how people explain sex to children - always good to get ideas.
With dd, it wasn't so much as explaining everything in one go, but more of a adding stuff to what she knew as she asked more specific questions as she got older. I can't remember when she first started asking, but I remember her asking why I sometimes wear a nappy like her, so she'll have been about 2 then. She insisted on calling sanitary towels "mammy nappies" 
She knew what sex was by about 6, we'd seen dogs shagging in the street and a few weeks afterwards she asked if its the same process for humans, the in out, in out stuff.
in the same school year a few boys in her year were discussing women's smelly pussies and other things they'd seen in some porn clip.
I wasn't prepared for that, questions about sex and how babies are made, yes. questions about porn and the stuff she'd heard the boys say such as "making her gag on his dick" I wasn't even contemplating at 6/7 years old.
I now know it's not unusual for boys that age to have seen inappropriate stuff, and that even though I monitor what she hears and sees at home, other parents don't, adult content can be blocked on your own DCs devices but there's a good chance that friends or older siblings of friends can show them content they shouldn't be looking at on their own devices.
It's not always intentional, the most innocent of search terms brings up porn, I know some of the parents of the boys I mentioned earlier were shocked, they were not aware of just how easy it is to access so hadn't set up controls.