well i think it would be ok i make baby quilts if i use flannel on front i put cotton on back or if i use cotton on front i put flannel on back. really a choice of your own

The thing about flannel is that the seams fray more easily. That is why the suggestion was made to use 3/8" or even 1/2" seams. The last thing you want is your piecing seams to pull apart from the quilt getting heavy use; wider seams with flannels helps prevent this.

It also depends on how much quilting is done. When there is minimal quilting, things like sitting on the quilt, hanging the quilt when wet, etc. will place more stress on both the quilting thread and piecing thread. For a quilt with flannel seams, it's a good idea to plan on at least a moderate amount of quilting to ensure there is no extra stress on seams.

The fraying seams issue doesn't come up with quilt backings, for the most part, because they tend not to be pieced. It's not that flannel isn't durable; it's that flannel seams are more likely to fray under stress.

I believe any kid would just love a new quilt, flannel or/and cotton. Most of the kids we are making charity quilts for don't have much in life and a quilt can bring someone much comfort. I would not worry about the different kinds of fabric. The quilt will be loved, no matter what.