The frame packs are fine, and probably a better fit than the Eureka internal. Just make sure they are adjusted properly for torso length. For the smallest child, you could look for the smaller Kelty Coyote pack, or REI has a kid's ultralite pack which might fit.

Either of the mummy bags from Campmor would be fine. The down bag would pack significantly smaller, though space usually isn't an issue with external packs. The Campmor down bag gets rave reviews for the price, though realize that the 20-F rating is somewhat liberal. It should be fine down to freezing, anyway.

Some other things to consider, thinking back to my own personal child in the backcountry:

Get basic blue-foam sleeping pads from Walmart for the kids. They'll need the ground insulation.

They'll need decent rain gear. A backpacker's poncho from Campmor is probably enough, or maybe a w/b jacket from Campmor or Red Ledge.

Synthetic clothing is mandatory except in very warm weather. Kids get wet, and they can get chilled and on toward hypothermic without realizing what's happening. (Been there, done that. Not fun.) Bring fleece hat and gloves even in summer (these need not be expensive.)

They should always have a whistle, and a headlamp or flashlight with them at all times. Teach them to stay put if they get separated from the group.

Good luck, and have fun.