New to chickens, need help with rescued Silkies

We have just started raising chickens. 3 weeks ago we received 11, day old, chicks that are Rhode Island Red/Speckled Sussex crosses. I was so nervous as I wasn't planning on getting chicks, but older established hens! They are doing great! The other day we were out looking at rabbits at a nearby farm and noticed they had some chickens so we asked about them and they were for sale. We had always wanted Silkies, so we bought 2. They are filthy! I am not sure exactly how old they are, maybe 3-4 weeks(one is larger than the other)? I washed their little feet today in warm water and put them under the heat lamp to make sure they got dry. But, the rest of their bodies are still dirty. I read that you can bathe them, but everything I have read has been for full grown show chickens. I will not put these in with our other chickens until we are sure that they are clean and healthy. They are eating and drinking well and their stools are good. It is just that they are dirty and I wonder if that effects their feathering out. Any advice?