Actually, anyone who uses the express is supposed to have a key card. We were faced with this on our first Unviersal trip when my youngest was a toddler. We ended up with two rooms, but at that time they didn't check everyones cards so it wouldn't have mattered (they do now, though). Of course a baby wouldn't be going on a lot of rides so it might not matter in your case. My son was old enough that we wanted to be able to bring him on everything that he was old enough to do (ET, Suess Landing rides, etc). When are you going? If it's a real busy time you might have a problem, but if not you could risk it. I wish that Unviersal followed WDW lead and allowed a 6th person in the room if they are under 3yo.
T&B