How do you tell parents they need to take better care of their child, without offending them?

*Back story*

I babysit a boy. When I went to pick up his spare carseat from them it was covered in dog pee. I cleaned it as much as I could but it still stinks. I am planning on cleaning it again. Not only that but lately I have been getting comments from people that HE stinks, I noticed his smell too but my scense of smell isn't that great so I didn't realize how bad it was. I am not sure if it's his clothes or him. I don't know how often they bathe him. Today, I had to take my hubby to work after I had picked up this boy and he said the smell was giving him a headache. I am not sure if it's something I should mention to his parents. Aside from bathing him myself and washing his clothes. They are going to be trying for another soon. And this is only one issue, there are many more but this is the worst at the moment.

*update*

I already talked to them about the carseat, I mentioned it right after I picked it up, the seemed to not really care so I cleaned it. As for the bathing their child, or washing their clothes, I took him to a mud pit on friday so I would hope that would bathe him. If they do not I will have to say something.