My husband moved out 2 wks ago. He has been in an adulterous relationship for about a month and a half. He now tells me, by email, that he would like to start incorporating the OW in my 4 yr old daughter's life. He says that he understands that I want him to take some time before doing this, but he feels he has. I live in CO. Is there anything I can do legally to keep this from happening? I'm going to sit down and try to talk him out of it tonight, for my daughter's sake. He doesn't realize what he could do to her in the long run if he follows through with this. He says that he is just making me aware of his desires. The man has gotten everything he desires thus far. When do I get what I desire? The selfishness makes me crazy!!!

From my SIL's experience, the answer is No, at least in IN. She was told there wasn't anything she could do to keep OW away from DS's unless the OW had a criminal record. I would suggest looking into OW background. I know here it's public record.
Otherwise talking sense into him is all you have. He needs to understand that your DD has to have time to deal with your H moving out. Maybe you should ask H to wait 2 months and you can discuss the subject again.

Are you considering divorce if so here is what I have done it is a hard road but if you have a good lawyer and a old school and moral judge it will work?

What I did was file for emergency temporary custody of my son and what my stipulations were
a. I have full custody
b. She has visitation with conditions
a. Every other weekends
b. Tuesday and Wednesday 4:30-7:30
c. And the big one she is not allowed to have anyone of the opposite sex with her when she is with my son (I told the judge that I believe that the child was at a impressionable age and that what she was doing was unmoral and it could hurt him mentally long term and cause unnecessary emotional stress on him.

Hope this helps but depending on your state you will have to file for divorce at the same time but some will do it on legal separation terms also.