As a childless 20 something, I'm amazed at everything you're doing. Seriously-sleepless twins who couldn't feed so you tried everything and got them into occupational therapy. And they don't sleep through the night, so you're running on zero sleep. And you're totally covering the bills, and are now looking for ways to reduce spending. You guys are amazing!
I second the suggestions to look into SoFi. Any interest savings will help! And redo your tax forms. A little more money coming home each month will help a lot. For secondhand items, google around for some shops in your local town. If you attend (or attended, pre-kids) a church/synagogue/mosque/etc., check there. If that doesn't turn up a lot, check for facebook groups. I live in a rural area, so my county has an "online yard sale" group. There's not a whole lot of kids' clothes because local organizations really work to collect those items to resell, but there's a lot of baby gates, kid bikes, toys, furniture, that kind of stuff. And it's pretty low key to check facebook while feeding to claim that you want an item instead of loading up the car to go peruse a store somewhere.