When you stay on Disney, everything is tied to that room key... you use that same CC type card to get into the parks, into your room, etc. Oh! And for the dining plan. And you *have* to use a credit at any of the sit down meals--you only pay for the meal, but you're really paying for the oppertunity to see all those characters as well. If you only have quick service meals then this isn't as big of a deal, but come on--part of going to disney is to see the characters, and there's no better way to see them than while you get to sit in a/c enjoying a meal. Yeah, it's expensive, and the food is OK, but it's just part of the cost, and you REALLY need the down time, esp. with kids.
Disney's hotels are not like a normal hotel room where you could sneak someone in. For goodness sakes--at the gate they check your passes to make sure you're staying on-site! So right there they might notice--I can't remember if they looked at ALL our passes or just the drivers, but I'm thinking it was all of them. And I'm fairly sure that Disney WOULD find out--maybe not on the first day, but sometime during your trip they would.
Like others have said, go to a different Disney hotel room that allows for 5, stay offsite, or don't go until you can afford a 2nd room.