Funny you should mention that. Just today, that very thing happened during Mass. I expect my almost four year old to make it through without using the bathroom. Almost two year old knows she if she starts to pull down her tights, I'll run her out.
Anyway, today, elder daughter announces she need to go potty. I tell her she needs to wait until after mass. She wails (rather loudly) "But I don't want to go pee pee in my panties."
So I took her out. I did remember that in the rush this morning, I didn't make her go and her pullup was dry, so she did really need to go. Chalk this one up to bad parenting.
I sometimes have one of the "old enough to know better" children ask whether they can go to the bathroom during Mass ... and then James hears it and yells, "JAMES WANTS TO GO TO THE BATHROOM!!!" and you have to take him to shut him up.
The offending child usually has to wash dishes for the rest of the day, because they know you can't mention the bathroom around James!
I think our Catholic mother stories have driven off the anti-Papist crowd.