Try Fox Hill. Does not have a pool for children as far as I know but it's luxurious camping- space for kids to run. If you want a hotel with babysitting services, kid's club and kids' pool (Westin Langkawi, Meritus Pelangi etc.), it will be above your budget. Try checking again later though, November is not peak period and if the resorts are not busy, sometimes they decrease their room rates to RM450 or so per night.
Langkawi is not exactly great for diving. I only know of 1 school (within the confines of the Yacht Club. If you are interested in that, Westin Langkawi is only 5 minutes walk away) but there should be more around.