Good raingear is a must, so are mitts and a toque.

Given your eldest is already over 5 feet, I'd imagine she's pretty much done growing and your safe with "adult" gear. As for the 13 and the 10 year olds, they'll probably grow a fair bit.

External frame packs are a wise choice because they balance the weight better. Internals are nice, but you have to pack them just so or else you'll end up with sore or off balance.

You'll want to make sure you have a sleeping mat, preferably the closed-cell foam. They keep you so warm, insulating you from the ground.

As someone else said, it's important they have a whistle and light, but I'd add a compass to that. Be sure they know how to use it and have them practice while they're hiking. That said, the should know the "hug a tree" rule---if you can't find the rest of us, hug a tree and we'll find you. Remind them that three blows signals an emergency.