I do think this walk is suitable for the whole family (but not pram friendly). Yes, you'll go up the hill and down the hill, but with proper walking boots, it shouldn't be a problem. Mum used her walking stick, and she said with walking stick, it made the walk much easier for her.
It's a coastal clifftop walk, took us 1 hr from Punchbowl Road car park to Half Moon Bay (probably a bit over 1/3 of the whole walk). Next time, we'll do the walk from Kilcunda entry.
The view is just awesome! We had "quiet" time on the beach@Half Moon Bay, too. The kids had their eyes fixed on the waves! They were just astonished to see big waves formed nowhere bashing onshore.
p.s.: the Punch Bowl Road off Philip Island Road isn't a main road, I couldn't see the road sign. So probably you may want to slow down when your GPS (if you use GPS) tells you it's about to turn. Car Park is at the end of the road.
There's no drinking tap, so you might want to bring enough water. You may have lunch at the lookout near the entry (100m from the Map Signboard). Either sitting on the bench or floor, you'll have a relaxing meal.
(Map at the Entry)
(Beach@Half Moon Bay)