I should have looked at this thread sooner. As a parent and grandparent who has always been on a low budget, I would like to contradict the advice about getting a bag that fits the youngster now. Children do grow, and those of scouting age are going to shoot up in the next few years! This year's bag that fits will likely be far too small next year!

It's easy to solve the "dead air" space problem by tying a cord around the lower part of the bag. I did this with my kids and also with my grandkids. Be sure to remove the cord for storage!

While a good down bag will last a lot longer, you're looking at a synthetic bag. Those have a life of about 5-6 years before significantly losing their loft (i.e. what was a 20* bag becomes a 40* bag). Even so, it's a good idea to get a bag large enough to fit the youngster during its useful life!