I'd say consulting a zoologist might be helpful in solidly identifying what kind of animal would drag large bones into its den.
Bones from a small child could not have been that large, apart from the skull I can't imagine any of those bones, even a femur, weighing more than a few ounces. Any rodent larger than a squirrel would be able to transport them. But you're right in implying that even if they could it's not just any large rodent that would have a use for bones as gnawing material, so to speak. Among the larger rodents (beaver, rabbit, marmot, nutria, etc) I looked up the only one that is known to gnaw on bones is the marmot, which also happens to be the only large wild rodent that readily dwells in or around human habitations, even in urban settings.
Indeed. None of the large rodents is a scavenger, they would not take bones from a fresh kill. They need dry, hardened bones. I wonder if teeth marks on the bones could ID the species? Knowing which animal brought the bones there could perhaps help in pinpointing the general area where they came from.Also, if animals are responsible, the bones could've come from an old grave.