My wife and I did lots of backpacking with our 2 kids since they were babies. They're now in their teens and not as interested, but we still do a trip or two every year and lots of day hiking. They're great hikers and they feel very comfortable in the wilderness. I just did Mt Whitney with my 15-yr-old daughter last summer, but it was a day hike. She kicked my butt up and down the mountain.
Like you, I can rough it just fine. Its a whole different mindset when you're responsible for little ones. The best trips were when I kept my goals in check, focused on their needs, and kept it playful.
Your gear ideas look right to me. The 20-deg bag rating should be right for the high country and fine for lower elevations, but its just a rating. Unfortunately, there's no universal standard for this specification, but loft is a good indicator, and stay with a name brand you can trust. Just as important is finding the right size bag. Good quality women-size bags can be found for 5ft heights, which might work for your 11-yr old. You might look for one that "hugs" with elastic to compress the empty space around the sleeper - check out the ones by MontBell. He can then pass it down to the younger kid. You'll probably have to find a kid-size bag for the 8-yr old. I had a hard time finding a high quality kids bag, but I found synthetic bags by Mtn Hardware and one by North Face, but no down choices back then.
If you're lucky, you'll have to decide between down or synthetic, but either way, I recommend taking extra precautions with a water proof sack for the bags.
I agree with the tent idea. I don't have any experience with the Copper UL3, but Big Agnes is a good brand and it seems reasonably light for 3. You might look for a little more room for gear and you might like the 4-person Tarptent
One last thing that you probably already know, the usual weight percentage rules don't apply to kids. They can't carry the same percentage of their own weight as adults. I never loaded them up with more than 10-12 pounds total weight at that age - just their own clothes and sleeping bag, water and snacks was about it. I made sure they each had a whistle and compass too.
Enjoy it while it lasts, it goes by so fast.