Kilmainham Gaol is a super tour. I'm not entirely convinced that the two year old would get much out of it, but if the parents have good English, then do it. €5.60 for an adult for a one hour tour. So interesting.
You can now hire bikes at the Phoenix Park for an hour or three hours, I think. Might be a nice way to pass a bit of time (if the Park isn't flooded, of course...).
You could get the DART out to Dun Laoghaire, maybe? Nice to be by the sea! There's a market there on Sundays. Go swimming at the Forty Foot (if the visitors are feeling brave).