Q& no A: One child has to go to the bathroom and the other doesn't

A situation that confounded me for a certain period of time, and is now plaguing an anonymous reader. I'll paraphrase (because the letter was very detailed):

"What do you do when you're out and your older child has to go to the bathroom but you're with a younger child, too?

I pack up the younger child with all his stuff and haul him along with us to the bathroom (and sometimes I can't even fit the stroller in the bathroom, depending on where we are). That's the best-case scenario. The other scenario is that the bathroom need comes on us so suddenly that I have no time to give a warning so the younger child freaks out and starts screaming and flailing not to stop playing and won't calm down when I say we're coming right back, so I can barely haul a kicking child with all of our crap to the bathroom and help my older one go."

I remember this, and it was my nightmare every single time it happened. My only real help is that eventually the younger one may be able to be convinced that when the older one goes, he should try to go, too. (One of my guiding principles of life is that when given the chance to use the toilet, one should. I'm passing that along to my kids, so they remind each other of it, and if one needs to go, the other one will try, too.)

What do/did you guys do? Is there some magic trick I completely missed? It's all the factors combined--the suddenness of the need in the older one, the resistance to change in the younger one, plus all the crap you have to haul around with two kids--that makes it so difficult.

Or do we all just grin and bear it until the kids are older?