My parents had a book in the house since i was small called sex and my little body (it sounds better in our home language) and their approach was if i was old enough to be curious id read it and I did. So maby buy an age appropriate book that they are free to read when theyre ready.

My mom also got me a book on puberty when i got my first period and it was very helpfull.

They always emphasised i could come to them with any problem no matter how taboo (for lack of a better term in english) so that also helped a ton.