Having 2 car seats in the back seat of my Forester right now, I can tell you you're definitely going to need something bigger than a Civic if you have any hope getting a teenager in between them.
If you want to stick with a car, then I'd try an Accord, Sonata, Fusion, Malibu, Camry. Although it's not my cup of tea, the Chrysler 300 is huge inside. Not sure how a new one of these would fit with your budget, but being popular sellers, I'm sure there's no shortage of these coming off 3 or 4 year leases.