4-H was what I was going to suggest too! But, in Australia you don’t have that. We had a book about chicks or chickens focused on egg development. It was great bc there were clear pages that could layer over the regular pages showing changes and development. It was geared to younger kids like yours. When searching Amazon, several options came up with “chicken egg development” and filters of “books” and then “children”.
we have this book, and my youngest has enjoyed looking at it and reading it. Yes, it’s meant for adults, but talks about several animals one can raise in a smaller setup, rather than large acreage.