I am a firm believer in one adult per child, if they are under 12 yrs old and 2 adults minimum. It is just as much for your sanity as thier safety. The big reason for 2 adults is that if you get hurt, the kids are not out there alone. If you cannot get a competent friend or relative to go with you consider hiring a responsible teen babysitter who has had some outdoor experience. By 12 years old, with training and experience, most kids can be quite competent and understand emergency procedures and not get too freaked out if something happened to you. Each child is different so these ages are just approximate. When I worked for NOLS, our junior courses were for 13-15 year olds and at the end of the course, with training, most were actually capable being very safe and survival savy.

Another savior for a bunch of kids is a horse-packer! When my kids were that awkard age (too large to carry on my back and too small to get anywhere on their own) I would hire a packer to take us into a drop-camp. We would then day hike from a base camp.

You may also consider taking one at a time until each is trained with one-on-one instruction. It is nice not to have all the kids be first-time backpackers.