when my DH started the CS process the court gave BM a hardship deduction for her first child. she said she wasn't receiving child support even though she was. basically what they did was reduce her income by $980 per month which required my DH to pay more CS to cover what she "wasn't getting" from the other daddy.
you might be able to claim a hardship for your child if your wife isn't working, but if both of you are working it would be unlikely.
even if she does ask for a hardship you can ask the court to deny it since she already had the child with you and knew her support amount before she decided to go out and have another baby.
wife of 1, mother of 2, stepmother of .3475902453