I personally have known many Jews that teach sex education in a mostly secular way to young children but then switch to a more halakhic/religious methodology when the children hit puberty. As a result of their influence I'm also planning on teaching my children in a similar way.
Some of these Jews gave us a good book recommendation for teaching sex education to toddlers that I feel is worth sharing. For those who may be interested in a completely secular sex education for young children I recommend the following book: https://www.amazon.com/What-Makes-Baby-Cory-Silverberg/dp/1609804856
The book teaches a dizzying amount of facts and presents them in a way that's easily accessible to children, while at the same time side-stepping the actual discussion about sex which you would then (hopefully) have at a later time. The book is solely focused on how babies are made and does not cover concepts such as sex, intimacy, homosexuality. But it does cover sperm, eggs, vaginal delivery, c-section, and miscarriage in a friendly and easily understandable way for a toddler to understand.
It can be jarring to picture teaching a 2 year old about sperm and eggs but we've found it to be very useful for our toddler. We're expecting another baby and our current child feels very caught up. Our daughter knows that the baby is growing in mom's uterus, and that the baby is made because I gave mom a sperm and mom gave an egg and we put them in the uterus and now the baby is growing.
This book is not geared toward teaching any cultural or religious norms, so that would still be up to the parents to do. The book is not a replacement for the very real and necessary teachings about sexuality and intimacy that is incumbent upon every parent to teach their child. For the teachings about sexuality and intimacy for teens/adults there are many great recommendations given in some of the other answers here.