If my child has to go "potty" then they have to understand. They have to understand that we will not be returning to where "daddy" and "sissy" are. When we return from the "potty" we will be going to the back of the line. If the child really does have to use the facilities, he knows that he will make the right choice by getting out of line (because when you gotta go you GOTTA GO). But if the child is bluffing, only so he can show you the toy in the gift shop located next to the "potty", he will not budge. To solve this problem go to the "potty" before getting in line. If the child is "throwing a fit" to get out of line, he will not ride that ride. He will just have to let the other children enjoy, while he sits with "mommy" on the bench. If he stops crying and wants to ride, he knows he has to stay in line. If he doesn't then you can just go home, while daddy and sissy have a great time at the park. If you give in, you are only telling him that screaming, kicking and crying will get him anything. This is something you have to be patient with. Start out with lines at the supermarket, then the local fair, then maybe the TTA at WDW, then you can graduate to slightly larger lines.
I have used this tactic with my cousins whom are now all well behaved "line standers." When I first started taking them to the theme parks 3 years ago (Kings Island was the first), we did have to leave early a few times because of the childrens behavior. Now they have all successfully been to WDW without incident. I developed and started this plan when I was 14 and they were 2, 5 and 6. I am now 17 and they are 5, 8 an 9. Once they were wretched little children and now they are heavenly angels at the parks. They know that no means NO. I am proud of them and their parents are too.
I know... I know... Why are we listening to parenting advice from a 17 year old? Because I was once a child myself, I knew and did everything to aggravate my parents. I threw tantrums and I kicked, eventually I got my way. You do not want a child like me. I am trying to prevent 18 years of hell for you. Just be patient with your children and let them know who the parent is. It will work.