Sorry I can't be more specific with HK-side schools. This thing is such a major deal for local parents that there are forums all over that discuss the finer points of various institutions. It really helps to have some insider knowledge, so if you have any local friends that could advise, or at least browse some of the Chinese forums for insight, it would be a great help to you. We got some help from our kids' Mandarin tutor. She was kind enough to find out about the various schools we were considering and gave us the local "perception".
In the end it made no difference to our application as we were allocated a school we didn't list (for various reasons) but it *did* help in the post allocation clearing in terms of which schools to reapply to (basically all the ones we had originally put on the form LOL). He ended up at a school that was number 2 on our original list ¯\_(ツ)_/¯
Another thing that may be worth considering if you are unfortunate to have to go through reapplication is that many of the better schools will hold interviews/assessments at a similar time. So you will need to weigh up your chances and prioritise accordingly.
In terms of a backup, other than your initial allocation you can consider schools outside of your net if you need to. We used to live in Taipo and had a place secured at a primary school there just in case it didn't work out in Kowloon. In the end we didn't need it, but there's no harm in trying if there is one nearby (but in another net) that might work.
The good thing is that once you have your oldest child in the right school, getting her brother in will be a breeze because it is pretty much an automatic thing for siblings.
Anyway, feel free to PM me if you need to ask me stuff. What I will say is that those two weeks of reapplications were the most stressfull, demoralising and tiring weeks of my life (and I have 3 kids!)