As others have said ... you need to do what you want!
Consider the length of use of the quilt ... being for a 7yo, it may live a life of TLC or it may live a drag around and into everything life! It may be wanted only for a short few years while she has that interest, or it may be one that is kept throughout her life and eventually used by her children.
You maybe just want to throw it together, anything goes. And yet, you may be wanting to make it with the care and respect as though it will be a heritage keeper.
We really can't tell you ... only you know what you want!
Some further thoughts ... a 7yo just may adore the quilt all the more with a minkee backing or a fleece. The "cuddlyness" of either could be beyond belief! And the colour choices almost endless. Each has their pros and cons.
Fabrics with polyester are more apt to pill ... again, is this really a concern that would negate the use? Only you know what it would mean to you and the owner.
For backings ... if you want a quilt to slide around a little more, a backing with some polyester will move more freely than a plain cotton or a cotton flannelette, as they "stick" more. More of a consideration for large bedspread quilts than for child size, as it makes it easier to make the bed when shifting a large mass around.