I agree with the prewashing. All that flannel, you might want to clean out the lint filter on your dryer half way through drying!
If you are afraid of unraveling, use a serger. If you don't have one, maybe a friend will let you use theirs. Or, do a small hem on the raw edges. Unless you can gaureentee that the quilt will never be washed, prewash!
I did a flannel quilt using the "brick" pattern designed by Liz Porter. It was real easy. The bricks were 3 1/2" X 6 1/2". No half bricks, you do stagger the rows, but wait until the top is done before trimming the edges. That was before I had a serger. I did not hem the edges but stood over a trash can and trimmed with pinking shears. It still unraveled some. Next time I will serge all the flannel.
I also had to clean the bobbin area every time the bobbin was empty, and that included lifting the bobbin casing out and taking a long paintbrush (like in a kids watercoloring set) and cleaning out all the fuzz!
I not only made the top in flannel, I used a thick flannel for batting and a solid navy flannel for the backing. My son couldn't get over how soft the quilt was. He kept hugging it - and he's a grown man - not a little kid!