Can someone recommend a complete guide for learning Arduino?
Arduino Cookbook (I have 2nd edition) is worth its price in paperback. PDF/etc formats are available. Children like to dig in without deep theoretical preparations. IIRC it covers classes briefly, as well as bitmath. A nice book. 

Boxall’s book is better still. A link: https://m.barnesandnoble.com/w/arduino-workshop-john-boxall/1114672111?ean=9781593274481

You don’t really need to do much except give the kids a laptop with no games on it, and a few arduino clones. Just lock the door to their room with a default-closed solenoid connected to an H-bridge. They’ll learn quickly.