Be sure the child is well past the age of accidents at night before getting a down bag. Synthetic bags are so much easier to wash and dry! My grandkids (who were rather late in that regard) had North Face Tiggers which worked fine when they were young. I don't know if those bags are still made.

A piece of cord tied around the excess length of a too-long bag will work just fine.
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May your trails be crooked, winding, lonesome, dangerous, leading to the most amazing view--E. Abbey