Prism99

Since you are quilting lines on the diagonal, I would say that the starching is a necessity because you are basically quilting on the bias of many fabrics. Heavy starching is especially beneficial for the backing fabric because all of your lines will be sewn on the bias of the backing. So, for this kind of quilting, I would not eliminate the starch. Heavy starching is also highly advisable if you are cross-hatching or in some other way crossing quilting lines. Without starch, you have a good chance of quilting puckers into many of the lines you cross. Next time, if you are quilting on the straight-of-grain, you could opt to simply spray starch the backing instead of using heavy starch. Spray starching is probably enough to prevent most problems if you are not quilting on the diagonal or cross-hatching. If you have a very good machine that feeds the layers evenly, you may not even need starch at all. Starch is like an insurance policy; you might do fine without it.

To soften up the starch in this quilt, you can squish the quilt around in your hands to break up the starch. This makes the fabric more flexible. However, I suspect the stiffness you are encountering is due not so much to the starch or to the spray basting, but rather to the batting you are using.

Warm and Natural is one of the heavier and thicker battings I have used. Because it is needlepunched through scrim, W&N is a very stable batting (probably the most stable batting on the market). However, it is that same needlepunching through scrim that creates a stiff drape in this batting. It softens up over time with use and with turns through the washer and dryer, but it definitely has the stiffest initial drape of any batting I have tried. I think that choosing a different batting next time will make for a much softer quilt going under the arm of your machine. A softer and thinner batting, such as the one the kids used, can make a big difference.

Edit: One other thing that might help you is ironing the quilt sandwich. You can do this with W&N because it is cotton; don't do this with polyester batting or you could melt the batting. Anyway, ironing the quilt sandwich after spray basting doesn't hurt anything (assuming cotton batting) and compresses the batting temporarily, which makes it easier to manage under the arm of the machine. With the first washing, that compression disappears and the batting springs back to its normal loft.