I did some vaulting back in the day, as well as helped organize some summer camp vaulting. I can't suggest any books per-say, just what I have experienced!
Ok, so my first suggestion would be to get a stationary barrel for the kids to practice on. Kids LOVE this (adults too). They tend to be expensive, but you can make your own. Get a barrel-racing barrel, weld it onto a stand and cover the barrel with some carpet or something that'll make it softer. You need to attach a surcingle or weld on some loops like the surcingle has too. If you do group lessons you can't expect beginners to all pile on the horse at once, and something like the barrel will give them something to practice on (or keep them entertained). It's also great to practice mounting/dismounting as well as raise the confidence of the littler ones.
Second, try out the horses yourself first. Some horses will turn foul after a while if whatever their doing is too repetitive. For beginners you want one of those baby-sitter types who'll put up with anything. Once everyone gets better you can move up to a more challenging horse. You also might want to try out the shorter horse first, people sometimes get scared with the tall horses. I learned to vault on a 17h and a 15h horse, and at the beginning the 15h was everyone's favorite. Of course, you can progress again when everyone feels comfortable =)
When teaching vaulting have a goal in mind. Maybe plan to have different "levels" and to do "testing" to level up. Have each level or group contain figures that are appropriate for the skill level, and upon being able to complete the figure the kids will "move up" Maybe plan for a small show. For lessons, also try to work in patterns and have a plan for what figures you want to practice and teach each day. Otherwise vaulting can get a bit random and you'll confuse the riders.
When I was small we also played games around the horse. We would stand in a line and when the lunge line passed around we would hop over it/ duck under it (horse was moving at walk) The most important part is to keep everyone entertained, including the horse. If you're just having fun then you can switch up directions too.
I hope this helps, and vaulting is really fun, so I hope you can teach it =) I'm not an expert, I just wanted to share what i've seen over the years =)