D.S.
My son (just over 2) wakes up at night thirsty also. Some nights he drinks a ton but other nights he barely sips. I've kept a cup in his bed for months now to help. I just saw a great tip in a parenting magazine for holding cups in a crib; get a cup holder that is made for the car (hangs on the door/window) and place the cup in there. If he drinks all of one the first time he wakes up maybe put 2 or 3 cups in there for him. I would mention it on the next visit to the pediatrician but wouldn't be too worried about it. Good luck, hope you guys get some sleep!