K.B.
My daughter was in a wedding last year and not potty trained yet (and even if she was newly potty trained I would have not push it on that day because it is a "new" and exciting event and no way she would remember to use the bathroom.
Anyway I got a cute bloomer to go over diaper, I can not remember where I got it but I know I went to JcPenny, Boston Store, Gymboree and Kohls (one of those places had it). If you can not find bloomers I would think white underwear would also work... thought the idea of writing something on them to congrats the bride and grooms is really cute wish I would have done that for the wedding my daughter was in. It would have to be place correctly to not be seen through the dress (or use a light color permanent marker, maybe one of the wedding colors if you know them).