A wide short board like this Starboard Go 170L (99cm wide) and small light sail - this is the Starboard 2m kids sail and matching boom/mast.
Flat, enclosed waters and light breezes really help a LOT. :-)
These short, wide boards, like the Go, are NOT just beginners boards that will be useless when you progress. The good ones are great light wind all round fun boards to use by more skilled kids and adults in lighter winds. I got my Go over 10 years ago for my kids to learn on, but I still use it regularly in summer light wind days to cruise around with a big sail.

The other suggestion I have is probably a lot less available, but it is by far the safest and most effective method I have found to introduce basic skills to kids, and anyone really.

This the Mk1 Starboard Gemini but I saw a newer Mk2 Gemini for sale just last week.