How about other book stores like Times, MPH, Borders?
BookXcess is really nice and prices are cheap. So far I've only been to the one in AmCorp Mall and SS15 Courtyard. Prefer the one in AmCorp Mall as it looks more extensive to me.
However, if you are OK with used books, you can also try joining those Facebook mummies group where they sometimes have members who either want to sell off books their kids do not want anymore, or even help you buy from overseas.

I would have to say, depends on your kid. If he likes pictures, get those with bright attractive colors lor. Or you can also get those "interactive" types where there are flaps to open.
I think there shouldn't be any criteria, different types of books = more exposure.