Did Drumheller that long weekend in 2006 so maybe out of date. The parks all demanded you pay the whole time even if not staying that long. We made reservations in March for that May at the campground in Beiseker which worked out great. Had 15a power and water and a sani-dump you can use on the way out. Not sure about playground as such--was a big grass field and a rec centre right there, could be all right for kids. Good family restaurant in town--yummy steaks.
Was a 45 min drive east into Drumheller and area to see the sights each day. ISTR about an hour west to Calgary.
It was an RV zoo in Alberta that long weekend starting about noon on Friday, all of a sudden everybody was on the road in an RV :) Half way through the weekend the mosquitoes came out to feast on all those exposed RVers!