Put the pants on to go there, but don't make a fuss about keeping them on, IYSWIM. It seems likely to me that the other mums will support you, I'm sure many of them will have been through it as well! You won't need to nag her - once she's gone once, it sounds like she probably won't need to go again until after she's had her drink.
We found it helpful to ask ds to have a wee last thing before leaving anywhere and (if we'd been out more than, say, one hour) as soon as we arrived. In the bustle of arrival he woudl often get distracted and forget, so we would simply remind him.