I've bought a series of 45 degree bags for my kids:
1) a Marmot Pounder
2) a Mountain Hardware Lamina 45
3) a Marmot Nanowave 45
They all got them when they were 5-6 years old. The Nanowave 45 is 9 and 50th percentile for weight. The other two childern are older and lighter. All three solutions have worked.
I like the Nanowave 45 because it is fairly light, it is a synthetic bag (not totally hosed if it gets wet), and it is cheap. I'll likely buy another one for my youngest, who is currently a lightweight 5 year old.
I haven't pushed the boundary of any of the bags limits to need to tie or fold over the foot. If I was ever that close, I just brought more snivel gear (fleece pajamas and maybe a poncho liner). YMMV. Listen to the other opinions.
As to colder weather, I've bought down REI sub kilo women's bags on sale which work for the kids, are more heavily insulated for their rating, and have worked well thus far.
Unless you can score a really good sale, the Kelty Cosmic 20 bag in a regular length is probably a good bet for $150. It's possible to do better with the application of more money or by doing better on the sales. It's what I'm suggesting to scouts - for now - until I see something better (suggestions welcome!!) and it's what I'd buy if I needed another down bag. I don't, however, own one so my opinion is based on the reviews. Speaking of which, you might find this: http://www.outdoorgearlab.com/Backpacking-Sleeping-Bag-Reviews/Kelty-Cosmic-Down-20