one thing i would do is have them read or study that topic during the week and then go out and do it on that friday, if thats not what you're already doing anyway
how to seal a
pond without buying a seal, do a small pond dug out with shovels and see what you can accomplish in a weekend
building different kinds of gardening beds and seeing what works and what doesn't, from there ask them to design their own hybrids with those beds to come up with some better ideas
not really
permaculture per say but you could also study survival skills and going outside with a pocket knife and seeing what they can do to survive for the night (if you have the resources in the yard that is), good skill and might be kinda fun to build a
shelter and fire for cooking/keeping warm in a semi-safe environment in case they ever need to do it in a not so safe situation
gorilla gardening:D
how to use plants for making things, such as taking an unruly patch of nettles and having the children make fishing nets from them
congrats on keeping your kids out of our oh-so-wonderful government subsidized education, i am in my senior year with an online private school, similar in that i am at home and have time for work, self study and an otherwise very busy lifestyle and i do wish i had made the transition much sooner