Your income isn't going to matter. They aren't going to force her to go to work, but they are going to impute income to her (to what she made before she left the workforce) and she is going to have to help support her son from her previous marriage, as she should.

Whether or not she goes back to work all depends on you, if you refuse to pay it then she is going to have to go back to work to pay it. You need to look at how much daycare is, if it is more than what she would make or if she would make just enough to cover daycare costs.