the doll quilt can be customized, size-wise, to the doll. So, if your granddaughter's doll "likes" a large quilt, make it 24x24. Your granddaughter is 8 and can cut the batting with scissors. she may be young for the rotary cutter, so I would suggest you cut the fabric. A rail fence is an easy pattern for a child. Simple strip piecing, to practice the size of her seams. Don't expect perfect, let her do as much as possible and then "fudge" it to quilt it if it isn't 100% square. She will love it even more if she does a lot of the work.