I love the touch of flannel for babies. I don't know if the flannel is strong enough as a binder, the edges get a lot of wear. If your quilt is already flannel, makes sense.
I still have a quilt I made for my daughter ( 25 years old now). The flannel is still in great shape, I know I used a regular store bought quilt binding. But, I was new to the quilting thing then... The blanket was used hard and then for her baby.
Just have the fun, enjoy you are doing for someone you know and they will love it anyway!
Chairjogger