I recommend the "Sexplanations" youtube channel for you to watch. Dr. Doe is a clinical sexologist and has a few videos on how to age appropriately talk to your kids about sex.

I homeschool my 9yo and we have used the “It’s So Amazing!” Series. I think the first book is for the 4-7 age group. They’re very thorough and inclusive books.

My nieces had a few books that were written in an age appropriate "how stuff works" way, with lota of pictures. How flowers make babies, hw people make babies, how chickens make babies. I even saw one that includes all sorta of diagrams about how pokemon make babies too. Just lays out the basics of how a sperm and egg, or pollen and egg, etc... work to make a baby, basic development of the seed or fruit or fetus or chick, etc...

I can remember the names of any of them though. But they seemed to be exactly the type of info they wanted, in the same age range.

Assuming boys are anything like my girls.. I answered when they asked. Obviously, speaking to their level of understanding, and most often when the other wasn’t there … to either be confused or take it further than the questioner could understand.

I remember their reactions being simply that of understanding, with them seamlessly moving to another subject ..to my relief. They’re resilient in so many ways, I think it’s actually difficult for a caring parent to mess that up

The right time is when they ask, so yours are ready. But the caveat is that it's not a 1-shot deal, it's ongoing. Only give them broad strokes that aren't complicated, and they'll continue to ask for more as they grow up.

I was 5 when my mom answered my questions completely and accurately. That did no damage to me, but did cause trouble when I answered the same question when other kids asked ME about it. Boy, did their parents get mad!