Yes, both can get in free on their birthday. You will need proof of their birthday which for kids is usually a birth certificate. I'm not sure if it can be a copy of the BC or if it has to be the real thing. I believe they also want to know their addresses (and phone numbers?) so I guess you should make sure you have that written down somewhere. You can also pre-register online but it wasn't working for some people but it won't take too long to register at DLR. Or if they have APs they will use that as their ID since they already have all their info in the computer.