As a handquilter who has been at it for more than 40 years I like good quality poly or a poly blend. It gives my handquilting defination, is usually easy to work with and lighter weight to handle. I make approximatley 20 Simple Linus Quilts a month. For them I use the 10 yard by 48" rolls of poly batting from JoAnns. I always stock up when there is a sale. I machine quilt them simply using a decorative stitch. I assume that children's quilts will be washed frequently and the poly does not shrink. I find that cotton batting tends to get very heavy with larger bed quilts. The king size bed quilt I recently won had cotton batting and it is a chore just to fold it up.
Just my two cents.