4 to 5 week old kitten not going to the bathroom?

Alright, my neighbor found these kittens under his van and was going to put them down. But instead we "adopted" them until they get old enough to get adopted to someone who will keep them. I estimated them to be about 4 to 5 weeks old, and according to a few websites, they should be going to the bathroom by themselves. But they aren't. I have no idea how to stimulate them correctly, but my male dog licks them a lot and got the one to go. He is very good to them. Unfortantly, the one hasn't peed much and hasn't pooped at all. I don't know what to do, should I let my dog keep licking it or should I attempt stimulation some more. The problem with me stimulating it is it scratches me a lot and I don't know if I'm doing it right and very afraid I may be hurting it if I try. Should I take it to the vet? Help please.