I bought regular size 40 degree Montbell down UL bags for my girls (8 and 12) when they were on sale and will let them grow into them. Went with high quality bags to keep weight down for them, and they absolutely love having nice bags like mom & dad. Yeah, it was a splurge for us, but having good quality gear helps to keep them excited about being out and enjoying the outdoors. Plus, I don't replace it when they grow. For our 8-year-old, I scrunch off the footbox as these bags have that feature built in.
Edit: For cooler weather, I have been watching for an opportunity to add to our family bag stash and order some MB#3 ULSS's for them in the future. These are bags that, if cared for, will last them into their adult life and I hope spurs them to continue backpacking after they have moved on with their lives in college and beyond.