Here is what I've done in the past with two:
- We use a lightweight rectangular bag (if you can find one), or another kind of bag that has the ability to "vent" at both ends to fit two toddlers: Their feet touch in the middle and heads stick out on either end with hats on. Obviously in colder conditions they would need additional head gear, but this has worked well down into the upper 30's inside a tent and it works out to be around 1 lb. per toddler with the old Marmot bag we have.
+1 on TNF Tigger - a well designed kids bag - not overly heavy like most and very durable for the weight. Use your REI 20% discount to get one, then pass it along on the gear swap when they've outgrown it. 1/2 zippers work great with children's bags.
- Sleep in between mom + dad under a double quilt or two bags zipped together (works when they're younger)
- I like down for the kids once they reach 5 +/-. I recently picked up a GoLite Adrenaline 40 short, which fits to 5'6", has 10.5 oz. of down fill and a snug fit that should work well for Summer+ use and a Sierra Designs Nitro 30 short, which is also a 1/2 zip snug bag with generous amounts of down. This should work for the remainder of the year in the PNW for the wee ones as they get older. Rather than tie off the extra space at the bottom of the bag, I generally "bunch it up" around them, trying to keep their feet in the footbox, but giving them a little extra down around them while they sleep. It works acceptably well.
- I have tried a couple of the Lafuma children's and short women's bags and been rather disappointed in the quality, materials, workmanship and fill of these bags. I try to purchase most of the kid's gear at either a retailer with a generous return policy (i.e. REI, STP), or buy second hand if possible.