Well shit, that's a predicament. We were going to keep the 3dr Yaris when my wife got pregnant. That is, until we tried to fit the car seat in the back and realized it touched the front seats when them all the way forward. So we bought the Camry.
Congrats on the kid! If your experience is anything like mine, the next few years will go by ridiculously fast, and there will probably be times you want to tear your hair out and go crazy, but they're worth it. Mine is in the "terrible twos" right now and is trying to his fullest to ensure that he lives up to the reputation.
I hate being rational and waiting, and saving, and making sure I don't have to make any compromises to buy the FR-S/BRZ... But time has seemed to go by way quicker ever since my son was brought into the world, so in reality it's not so bad.
Anyway, if you can swing it just get something with four doors and practical. SUV, sedan, wagon, whatever. It just makes life easier while they're rear-facing. My advice for practicality? Sell everything you own and guy buy a Ferrari FF. Remember, you can live in your car, but you can't drive your house to work!