How can I tell my parents that putting guilt on their 2 1/2-year-old granddaughter just isn't going to make him any more eager to use the potty!?
If you and your son see grandma and grandpa frequently, you'll need to take them aside and tell them as lovingly as possible that you're managing potty training your daughter and to please not make any comments (offer examples) that are guilt-provoking.
Tell them that your focus is to offer encouragement and to expect her to practice knowing that in her own way and in her own timeframe, she'll learn to use the toilet. If you and your daughter only see grandma and grandpa occasionally—like once a month or so—ignore their comments, as they'll likely have little effect on your child learning to use the toilet.