I have only bought the dining plan when I booked the whole pkg through Disney. However, I read a lot about it here and it seems you can request a "ticketless package", which just means room + dining with no tickets. Some phone CMs may not know how to do it, so the phrase "ticketless package" and possibly asking for a supervisor are key.
If you haven't investigated the pros and cons of the plan, be sure to check out the dining plan discussion board here - there's always at least one thread going about the pros and cons of it. With 9 year olds, they are still kids on the plan, which can be a good value, especially if you're going to do several character meals or other fixed price meals. Depending on their tastes and appetites, you may not find it a fit though, since kids must order off the kids menu at table service places that have one. My kids at 9 wanted a bit more food than that, and preferred the variety on the adult menu.