It is okay. Mother Nature has a way of doing this so we only have the strongest and healthiest birds in our flocks. It is sad and we all hate when it happens, but it does happen. We make mistakes raising them and hopefully we learn from them.
In answer to the pitbulls KEEP them away...
1 I change to grower as soon as they are out of the brooder. And then change to layer about 16-20 weeks. You will get a different answer for ever person you ask.
2 As far as a dust bath I mixed playground clean sand, DE, and woodstove ashes. However they enjoy uses just the dirt...