Vireo UL

Yes I think that's it. Picked up about 10 years ago... it's not exactly a high end bag but it does pack relatively small I also have a feathered friends Vireo UL (hoodless zipperless down mummy) but that doesn't sound like a good fit for your use case

I backpack with my 9 and 6 year old boys. Last summer, I had them both in 12L REI Tarn packs that I picked up on Facebook marketplace for like $40 combined. They use 8 and 6 sections of Z lite foam pad. I bring quilts for them, one of which is an old 28 oz 10 degree EE quilt from when I first went ultralight, and the other is a Feathered Friends hoodless sleeping bag that I bought for basically half price on Facebook marketplace this off season. Just make sure to bring a warm beanie for the kiddo to keep his head warm. 18L is about the most volume I would recommend in a pack for a 5 year old. You’ll want the pack light and small enough that you can easily carry it in an extra hand if he gets tired of wearing it. The torso size (assuming the pack has a hipbelt) is one of the more important factors at that age. Ideally, you’ll want a pack with an adjustable torso length, th shorter the better at his height. My 4’4 9 year old has about a 12-13” torso measurement for reference, but it’s easy to measure. Bottom line, you’ll end up with some extra weight and volume in your pack in order to make the experience better for him. I

I worked at a man camp for one winter. Used a down 20 degree sleeping bag most of the time, but also had a second bag to double up when it dipped down lower. A sleeping bag is a lot warmer than a quilt. I slept through a few -20F nights without much issue. I had a lightweight Feathered Friends bag, but I recommend a heavier weight material for hard use if weight isn't an issue. I remember fixing a hole or two with some duct tape. I still use that same bag.

Feathered friends will customize bag sizes. At 950 fill power down they are as lightweight as you can reasonably get.

The ratings are not always reliable so you are right to be uncomfortable with them. One good baseline is ounces of down at the same fill power. Comparing bags that way gives you a better comparison point. Western Mountaineering and Feathered Friends are generally reliable in their ratings. That should give you an excellent place to start.

Feathered Friends will customize bags. They did it for me.

Western Mountaineering, Feathered Friends or Marmot for bags. Enlightened Equipment, Hammock Gear or Katabatic for backpacking quilts. Not cheap but quality and comfortable.

I've been a quilt user for years, but if I were looking for sleeping bags, I would go with either Western Mountaineering or Feathered Friends. You absolutely get what you pay for with either brand. Awesome quality.