Im my experience, every horse breed is different and there will also be the quiet horse and the hypo horse of every breed.
I think its important you ask yourself what discipline do you want to expose the people to? Western or English riding?
Buy horses suited to that discipline. There's no point buying an english horse, and then putting a western saddle on his back.
I find paint horses good to deal with in general, and cobs are also alright. But please bear in mind that there is exceptions to every breed.
Also, a release form saying that horses are dangerous even life threatening should be signed by every participant, just so that your outta hot water in case anyone gets hurt.