Fantastic bow- your niece will be one very happy little archer!
You might want to consider making arrows for it which are just over the 20" draw length. That will help prevent older or bigger kids from overdrawing and damaging a such a great bow. For the 20# draw weight, they'll likely be too stiff so, consider full length tapering them to 7/32" plastic nocks. Here's some 23" ramin arrows made that way: