Just noting that my 3yo weighs around 35lb and she is HEAVY to carry for short distances. I'm not sure any normal quilt battings would do really well in a quilt that weighed that much. I figure the weight would just flatten anything you put in it. I'm also not sure what that would do to a flannel backing, as flannel tends to be a little stretchy, in my experience.
A denim top and backing would be pretty heavy together, but I have no idea how you would quilt a monster like that. Perhaps that would get you the weight you wanted though?