Wow, someone else has my same problem. I have a 6,5,2 and 17 month old. So far, the wife is not willing to go with the younger two. When we do go day-hiking, it isn't long before I am carrying the two young ones, and the problem is that they are getting heavier, not lighter. <img src="/forums/images/graemlins/grin.gif" alt="" />
I would suggest that you make the older two carry their sleeping bags. I just finished making my 5yr old a pack that fits him perfect, and can carry his sleeping bag ( I haven't found a commercial pack that does this). Probably 5 lbs total (Pack, bag and pad). I will make one for the 6 yr old as well. Then I only carry my bag, and all the other gear. Also, make them carry their own pad. Give them just a cheap blue ccf that is cut to their height. It will weigh almost nothing. This will also reduce a huge volume requirement for you. If all they carry is their bag and pad, it will hardly be any weight. I would also make a quilt for the two young ones, if not just have them sleep in your bag with you. That way, you know they are warm, and you reduce your bag load.

Good luck! <img src="/forums/images/graemlins/grin.gif" alt="" />