We dealt with this very early on. DD would wake up in her crib, and we'd do all the usual checks for hunger/wetness/chilly/illness/other distresses, and when it became obvious that all she wanted was to play, we'd just tell her good night, and go back to bed. Later when she was more articulate, and she wanted to stall by asking questions, we'd just say, "Ask me in the morning." If she pressed, we'd say, "Now is not the time for questions. Now is the time for sleep."
If you're getting up with your child and playing with them for two hours in the middle of the night, you're basically encouraging and rewarding this behavior. Why would they sleep at night when they can play with mommy and daddy?
