I think you will find everywhere in HK is a stroller nightmare!!!! There are lots of steps, not a lot of pedestrian crossings, and buses that don't accomdate even the smallest of strollers.
Out of your three choices I would say CWB was the worst place to be with children....it has the worst recordings for air pollution in HK, is very congested with people and traffic and on weekends you can not move for the people there.
Happy Valley is much quieter....and you have the grass round the race course where you can take children to play, but the public transport in and out of Happy Valley isn't great, but this wouldn't be an issue if you plan to take taxis everywhere, or walk!
Mid-Levels can be ok for kids....if you choose an apartment in a building with facilities, i.e. a pool, playground, indoor playroom. There are plenty of people living in Mid-Levels with kids.
I personally wouldn't choose to live in either of those places with children....hence why we live in DB but we all want different things from the places we live!