You should be ok in Sai Kung, but around Hong Kong Island and parts of Kowloon, you're going to have a few problems. Also keep in mind if you plan on taking the stroller to the supermarket you might want to do a bit of recon first to see if the aisle are wide enough as many supermarkets don't have the room for large shopping trolleys.
Having said that, walking the waterfront with the stroller in Sai Kung should be quite nice (although Sundays can get a bit busy).