Thanks for the replies. I have changed plans and now am going to stay in Conwy instead. I have been to both places briefly and thought Rhyl would be good from a 5 year olds point of view, but some of the reviews here and elsewhere on the web, plus my recollection of the place, plus the fact that it will be Guy Fawkes weekend kind of helped convince me that Conwy would be a better bet. It may not have a fun fair along the sea front, but it does have a cool castle (with a railway station in the middle of it) and other things to keep us busy. Only problem is that the train service out of there on a Sunday is pretty non-existant, so I reckon we will go down the road to Llandudno Junction and get the train back from there.
Cheers,
z