Oh I looked into it recently and discovered endless os, it has from scratch parental controls, an offline encyclopedia/Wikipedia lite an other educative softwares and games. You can use it totally offline as it seems to be made for educative purpose. Check it it could be interesting for your purpose. (You can also download and install more stuff for it of course)
It's freeware of course. Their installer took ages to download, there are torrents of their full version (12Gb)
I think the added packages are just preinstalled software from their curated store (only family friendly and educative software) but that is just asupposition.