That is what we had this past Wednesday. My 2yr old daughter and 2yr old niece both know when they need a change, but they refuse to tell us when they need to go. They know when they need to go, but they would rather do it in their diaper, in their underwear, or all over the kitchen floor (and kitchen chairs and bedroom floor).
So, as an incentive, we decided (my SIL and I) to throw a Potty Party for the girls. We figured it would help give them some "sweet cousinly encouragement". They sat and played and pee'd and sat and played and pee'd. We tried EVERYTHING! We had them sit every 15minutes. They would sit for about 5-8 and then pee all over the floor in the bedroom. So we extended the sit time to about 10minutes. No success. So we decided to keep them on the potty until they pee'd. Then we got smarter and made them sit with their cups! Ha ha, look at us go! But, look at them not going! So we stuck a movie in front of them and let them have their party right on the potty! They pee'd! They pee'd alot! And they had special potty charts (which they just loved!)
I was able to get some really cute picture of the two of them but am hesitant to post them b/c...well, lets just say, they really are cute in their birthday suits! :)