I can understand your frustration with the child support being paid and your budget already being made out and spent. However- as a person who has lived it and went through all of those frustrations and now being in a family who hasn't seen my husbands bio daughters in over 4 years- I have to tell you now- the pain that your DH will go through, the pain his kids will go through, the pain you will go through, AND you own bio children will go through from not knowing their siblings is not worth ANY dollar amount that you could save from only buying them small things or nothing. Those are your husbands children, HIS blood, as much as your own child(ren) are with him. Some of the previous posters have excellent ideas about returning a portion of the gifts already bought so you can purchase gifts for the other children as well. It isn't about the gifts it is about cherishing EVERY SINGLE SECOND that you can get with those kids. EVERY SINGLE ONE!!! Regardless of what you think the ex's motives are- be THANKFUL that you and DH can see those kids and take that small amount of time to get to know them, take photographs, love on them, and BE a positive influence. If their mom truly is such a bad person, those kids will need their daddy someday. Let their memories reflect a happy time when it is spent with you and him and their siblings.
I would give heaven and earth for my DH to have one more minute with his girls. Please- cherish those kids!!!