Ditto for what the other mom said about in-home daycare. Try checking into Head Start, too...they offer preschool and it's either a sliding fee or free, depending on how much you make.
As to saving more, clip coupons, and use them when the product is on sale...don't buy it if it's not on sale. It may take three weeks to get what you need for spaghetti, but you'll save a couple dollars.
Also check out Flylady.net. They have different ideas to help save $, but also provide a lot of support, too.
As to the second job, see if you can find another mom to do a babysitting swap...you watch hers one or two nights, and she watches yours the same...might get you a few hours of extra work, or even some peace for yourself without having to pay for it. Good Luck!
(Btw, I'm a 37-YO single mom of 2 ds, ages 3 and 5, so I know what you're going through!) :)