But the question is how will they know?
I don't think they will know. If you don't do EMH, they will not know. As far as the room key, if you guys are usually together, it only takes one key to open the door, right? But you have four keys available. The two children can do without. Or one child can do without the key.
Depending on where you're staying.. some resorts do not ask for the room keys to swim at the pool. Of course if you're staying at the popular resorts, where they have popular pools such as SAB... then they will more than likey card you, which in this case, you will need a 5th key, unless they are not planning to swim.
So... how will they know. They won't know. If you're asking as to why Disney has limits on guests per room... that is another story. But you didn't ask that.