It's been a few years since we were at Hopewell Rocks in NB, but I know we went early in the day to see low tides, parked in the Visitor parking lot and had lunch, etc. and waited for high tide to come in. It really is spectacular to see both high and low tides. We were able to walk around the rocks when the tide was out, but I am fairly certain the visitor center has a large viewing area where you can view the tides. It would work for you in-laws. Parking a large rig should not be a problem.
A hint - if going to PEI, take the ferry from NB over to PEI and come back on the bridge. The ferry charges by the foot whereas the bridge is a set fee for RV's.
Sorry I can't help with the tidal bore - another tourist attraction but not in the same area as Hopewell Rocks.
On PEI, be sure to spend some time in Charlottetown and visit Founder's Hall. Also the play, Anne of Green Gables is enjoyable for all ages. It runs until Sept. 23rd.