Since you are done having children, how about selling your child's toys, equipment, and diapers as he outgrows them? Craigslist is a great way to sell and get $25 or $50 here and there. For clothes, if you're not already passing them on, you can go through them and sell them at a consignment sale. I regularly do a fall one and spring one and make $100 at each one. It's not a lot, but it's fun and it helps.
I also think you are doing great. One suggestion - try to cut down on the Target runs! They will eat up your budget. Make a list when you shop and stick to it or try to shop somewhere else that isn't as tempting.