i have a friend who is an outdoorsman, both out of interest and profession. whenever i have an expensive or uncertain purchase to make, i reach out to him and each time he answers. virtually every time i hear back from him (as i try not to pester him too often), i'm astonished anew at his breadth of insight and depth of his thoughtfulness. being a collector of thoughtful objects i wished to add this to my collection of things. i also surmised others would appreciate and benefit from his insights. so with his permission, i share the following about hydration systems, packing for a hike, and approach to kids.
Summer's been great so far. I've been enjoying the cooler temps, although the kids have complained that "it's too cold to go swimming". They'll be happy that it's heating up.
We have just used the many packs we have accumulated over the years, or the kid's school packs. I have a couple of Camelbak bladders that we pack along and fit them into whatever packs we end up using as well as water bottles. So far our hikes haven't been long enough or varied enough to require that the kids carry their own gear. Typically Jona handles the food and I try load up with all the other group necessities--sunscreen, bug juice, sunglasses, lip balm, first aid, head lamp, etc.--it all adds up.
Getting the kids to carry their gear is a great idea. I think you probably have packs already that you can use. If not find some that fit each kid well, are comfortable and are as light as possible. There are some super light/minimalist packs out there now that work great for carrying light loads. You can help the kids get excited about carrying some of the things they want to have along and they are helping out too. The important thing is to be the one that "shakes down" and explain/enforce what they pack so they really only have items that will be worth carrying along or won't become a needless burden along the way. Kids being kids usually will insist they pack along items that aren't essential, will be forgotten about anyway, and will start feeling pretty heavy after awhile. Also group redundancies can possibly eliminated--does everyone have to have their own camera/binoculars/etc. or can the group share them? Maybe agree to limit only one non-essential item per child and make sure its something that won't be a problem to have to carry.
Camelbaks are great for a couple of reasons--first water is one of the heaviest items you have to pack. Spreading out the weight of the water along a greater surface area and close to the back makes it easier to carry. Second drinking through a drinking tube means you can have a sip whenever you choose. You don't have to unscrew a lid, you don't have to stop, you don't have to wait, you don't have to even think about it--just put the tube in your mouth and drink. That means that everyone tends to drink smaller amounts more frequently which helps them regulate their own hydration and stay hydrated a lot better which is obviously super important.
Kids get used to drinking from Camelbaks pretty well, but one caution is that they are alot easier to breach in some way by excessive fidgeting or playing or poking, and then the water is gone and maybe the warm fleece jacket packed along is now soaking wet. You don't have to buy a complete Camelbak (or other brand) backpack, you can just buy a water bladder and insert that into any backpack.
The greatest consideration is just being sure that whatever packs you choose, and whatever is packed into them (including the water), they remain super light, comfortable and the weight is an appropriate amount for each kid. You want them each to have fun, enjoy the hike and be able to make the distance, and everyone is much more able to do that if they are not burdened by struggling under too much weight. Besides, you're the one that will have to end up carrying the stuff, or carrying the kid if it all goes south.
Don't skimp on the water even if you have to carry it all yourself. Make sure there is more than enough for everyone for the conditions/location you are in. Sometimes you have to pack it all along; sometimes you can refill along the way. If you refill, make sure you are treating the water appropriately from any natural source so no one gets sick--water treatment is a whole 'nother topic!
Good questions. Let me know if you have others or if I can help in any way.
Enjoy the hikes!