Hi - we are also staying at BCV in Sept. by renting points from a DVC member (we are a family of 4 with 2 kids ages 8 and 4). We asked our member to add the
DDP to the reservation, and we will pay for it at checkin. My understanding is that
free dining is available only with a package purchase (at minimum, room + some form of park admission).
The way I approached the decision to purchase DDP or not was this: DDP will cost us $100/night. Before DDP, on a weeklong trip we ate mostly counter-service + snacks and a couple of the lower-priced TS (Boma, Biergarten, San Angel Inn, Kona Cafe for breakfast). Including an offsite grocery store run for in-room breakfast foods and libations

we still spent nearly $100/day! We would be happy to "do WDW" that way again (i.e., mostly CS) but now for the same amount of money (probably a little more since we will still have some OOP costs for the grocery store run, etc.) we will be able to eat TS meals at places we never would have tried because of their inflated prices, and on top of that we can order whatever we want off the menu. So that will be fun. I look at DDP for us not as a big money-saver but as a way to add, for no or minimal extra cost, an extra element of fun to our vacation.
As for your other questions... Does a TS every day take away too much time from touring? Obviously, that depends on your definition of too much. We do like to maximize our touring time so our general plan is to get an early start in the parks, stay away from sit-down/character breakfasts, eat early CS lunch to avoid the rush, take a mid-day break, use our TS at dinner, then stay out late in the parks. I see you have more and younger kids than us, so the stay-out-late plan might not work for you. The good thing about Sept. is that while the restaurants may be crowded, the parks shouldn't be, so efficient touring is possible.
Hopefully we will not lose a lot of time waiting past our ADRs to get seated. Reports are all over the map on that - some people report never having to wait, others report isolated instances of having to wait up to an hour. Since that is something we can't control or predict with any accuracy, we'll just hope for the best.
Good luck and have a great time!