If they have never been before...I suggest start with the outside rides first like Dumbo, Casey Jr, Tea Cups, When you move to the inside rides, try IASW first, then do some of the scarier ones like SW, etc.
This works well if the LO's tend to get scared.
We took DD (3.5 at the time) and my niece (a few days shy of 3) last year. We went straight for SW and some other FL rides. My DD was a little scared, but ok since she knew the stories. My poor niece was so scared (especially after going on pirates) that she can't even go to the movie theater because it is dark. My DD rode anything and everything she was tall enough for. At the end of the day, she wanted to see the robot movie (Star Wars) because that was her favorite ride...still is.
The princess M&G and JTA will be fun if that is what they are into. They may also enjoy Toontown...just check the seat before you get on GGC.