A bit of a tangent, but you might not need a "monster pack". Kids can carry some light, but bulky gear, which reduces the volume requirement of your pack. I've found the kids prefer to carry some gear.
At 3 years, my youngest son carried a sweatshirt and a roll of toilet paper–we usually take a full roll when traveling with kids. (A kid that small isn't hiking far even w/o a pack). We made a big to-do whenever using the toilet paper: I need the toilet paper. Who packed it in? Oh, right–in Sammy's pack. He was a proud little guy.
At 4-5 years they can carry more and go further and I've had them carry a sleeping bag (I use MYOG down blankets for the small kids). They can also carry other light/bulky gear like a titanium pot/stove combo. Trust me–if your kid is carrying the pot and stove, he/she will feel pretty important.
Packs for kids are NOT light (ranging around 2 lbs), so your kid's pack weighs 2lbs. He/she carries a 12 oz sleeping bag and the stove. That's 4 lbs total. The child will want to carry his own water and some trail mix, and have a small LED flashlight. A trashbag ponch can completely cover a kid and is almost weightless. Assuming a 40-50 lb child, you're hovering right around 10% of their weight, which is very doable.
This leaves you with the heavier, more dense items, which is generally food and tightly packed shelter, poles.
I've never had to carry a pack larger than the ULA Circuit, but I use low volume shelters like the GG Spinntwin and pyramids. Now that my kids are bigger and can carry most/all their gear, I don't carry the Circuit–it's too big. I'll probably sell it here to raise some money for a new shelter.