These kinds of accidents are part of every parent's camping experience I think! What has worked well for me in the past is to use an enzymatic cleaner like Rockin' Green Platinum Series Active Wear Detergent. Make a solution and apply it liberally to the affected area. Let it soak in for a little bit and then clean with water.
Hopefully that gets you taken care of, thanks!
At REI, we believe a life outdoors is a life well lived.