Hey Eric- Our family of five was considering a Tacoma 4 door as an addition to the current 'fleet'. We've test driven them a few times and done a bit of research as to whether we'd fit. Right now it's three child safety seats across in a 4Runner, but by the time we would have been ready to purchase a Taco, two of the child seats would be optional. All three of our child seats fit fine in the Taco after all. I'd figure the kids would have enough leg and 'mom! He's on my side!' room well into their teen years. They were really disappointed we didn't take it home with us. (They keep talking about 'when' we go back and pick up 'daddy's new truck'. So it was a quick sale with them!)
We also compared it with the Tundra access cab (this was back in early summer 2003, and prior.) I didn't find the back seats in the Tundra to be as comfortable as the Taco. They were stiffer and more upright, and a turn off immediately. You might compare the 4 door Tundra, I haven't been shopping for a while so I've no idea what the seats are like.
Best bet for you is to take both for a test drive with your family. See how you all fit (even if with the salesperson, one of the adults has to stay behind at the dealer.) Then decide whether you NEED the bigger truck or if it's just a want. My long-legged brothers and I grew up shoe-horned into a little Honda station wagon and did numerous enjoyable family vacations in it, so unless you're just big people, the smaller truck is probably enough space. If cost is a factor now when looking at which to purchase, then it's probably a factor when it comes to maintaining and operating the new ride. Which gets the better mileage? Which is cheaper to service? (I always look at long term costs, don't like to be sidelined by unexpected bills.) If the reason(s) behind the new seating 'requirements' isn't arrived yet, borrow a baby seat from a friend and see how it fits, or have a friend with kids come with you to look at the vehicles, see how well they fit, including diaper bags and other accessories. ;) It's a major purchase, so treat it like you're shopping for a house, not an entertainment system. Think of how big your family might get, whether they'll have bigger toys later, etc.
I'd jump at the chance to own a truck, especially a 4 door Taco, but we really have no need for one right now. The 4Runner covers our needs perfectly. Cost for us was another factor, as was lack of anyplace to park it here. We're searching for a certain classic show car to buy in the near future and with that, hubby's hobby/daily driver, and my 4Runner, there would be no room for a fourth vehicle. I urged him to decide between a new truck and keeping his current car to mod out to his heart's desire and also getting his hot-rod. He's still not 100% sure which he wants to do, but he's started planning mods so I don't think the truck is in the picture anymore, LOL.
Good luck deciding!