H.W.
I'd clean it thoroughly with a bleach solution, twice, knowing me. Mouse feces can spread deadly viruses which can be breathed in. I'd want to know that it was totally contagion-free before passing it along, just in case some other parents aren't as informed. I'd hate to be the person who dropped the ball on that and have kids get terribly sick.
Then, I'd probably tell the truth: that it was exposed to mouse droppings and what I'd done to clean it. Just because someone gets something for free doesn't mean they want to be kept ignorant of a potential hazard.