If you get base tickets you can always get the minimum tickets you think you will need and you can upgrade your tickets at Guest services should you find you need to "hop" or go to a waterpark. Also consider your day of arrival/day of departure. If you fly in late or fly out early, you may not be able to go to a park on arrival/departure days. So the minimum days you need may be 8. For such a long stay you probably will want to hop (go to more than 1 main park in a day). But hopping wastes time and if your kids have never been to WDW there may be lots of things per park that you want to see. The hopping option is I think $55 per ticket which is not a lot but for 5 tickets that is $275. One trip I bought base tickets and figured we would upgrade at guest services if we needed to hop. We didn't so we never upgraded. But if we had purchased the hopping option and not used it, we would not have gotten a refund. Same goes for the water parks. You might not add that option upfront and just add it when you know for sure you will be going to a waterpark. Although I believe if you have the hopper option already, the waterpark option is only $22 more. But you can never predict the weather or if someone gets sick and maybe they won't feel like going to a waterpark. Plus, if you stay at the Beach Club, that's a great pool. Your youngest may not want to go to a waterpark so one parent may stay w/ the youngest at the resort pool while the others will go to a waterpark. But since this is an employer paid trip, do you need to spend up to the $10K? If you don't spend it all, you won't get it back as cash, right? For sure don't get single day waterpark tickets. They are around $50 per park per person. And as mentioned before, to add the waterpark option to your park tickets is only $22 per ticket. And you would get up to 10 waterpark visits for that $22 (if you get a 10 day ticket).
If you're going to be staying at BC, I think I might hang off on getting water park tickets in advance. You may find that SAB is all the kids really want. You can always get single day tickets later if you want them.

