The class I go to (when I can get a sitter for my 3 y.o.) is on Thursdays at 11:00 AM (1 1/2 hours). I think it used to be at 10:00, but they found that was too early for people to get there. Ds doesn't have any set nap times yet, so that's not an issue. I don't recall it being a problem when I used to go with dd. She slept, nursed, cried or just hung out.
Are you planning on involving the babies in the class or is it mostly just for the moms to be able to bring their babies? The one I go to is strictly for the moms, although it is very relaxing for everyone (Kundalini yoga) & I'm sure the babies enjoy it too.
I think weekdays usually work best for new moms since they usually have dh home to help out or do something as a family on weekends. Especially for first time moms, it gives them something to look forward to during the week.
Not sure what a workshop would be as opposed to a class. It might be nice to have the dads in on that one just so they can see what's going on, although I think the pre-natal one I went to was better for that & gave dh some massage tips. I don't see a problem with having classes open to dads, but I also don't see too many dads being interested in something like that (unless you are going to be doing exercises involving the baby that they can learn to do at home).
I'm also wondering why you are putting an age limit on being able to start in the class. When dd was born, the first three months seemed to last forever...I think I would have gone crazy waiting that long to go back to yoga. My pre-natal instructor always said you should stay home & bond, etc. for 40 days (&, to be honest, I felt no desire to do anything before that with dd), but if someone wanted to come to class after a month or so I don't see why that would be a problem.
Helen Homeschooling Mama to Nicola 07/00 , Daniel 05/03 & X2...and with Barry since 08/87