The discount comparison also depends, to some degree, on whether you would purchase the dining plan if it's not free and, if not, how much you would spend on food without it. I've seen people post sample daily menus comparing the cost of what they would plan to eat with in order to compare with the cost of the dining plan. I'm too lazy to do that though. It's all I can do to think about making dining reservations 180 days before my trip. I don't know what I want for dinner tonight, let alone in 6 months.
Anyway . . . your family sounds a lot like mine--2 kids, one who Disney sees as an adult (and so does she, but that's another story)

We've done the dining plan twice before, once when our younger dd was under 3, so she was a freebie, and once when both kids were Disney kids. Also, back then appetizers and tips (I think) were included in the ts credits. Seemed a pretty good deal to us. But with a 12yo "adult" who is a very picky and light eater, it becomes less appealing. Then again, however, we are planning a couple of character breakfasts, dinner at `Ohana, and a Candlelight Processional dinner package where she will be charged as an adult regardless of what she eats, which makes it seem a bit more reasonable. I haven't done the detailed math and menu analysis though for the following reasons . . . .
We are currently booked for 7 nights with
free dining. The dp would have cost us about $1050. Rack rate for Poly for those 7 nights is about $3300. My thought process is that a 30% off code would be about a break even that might force me to get more serious about calculating what we would expect to pay out of pocket for meals so that I could figure out which is the better deal. Seems like FD is easily a better deal than a 20% off code, but if I could get my hands on a 40% off code, I'd drop the FD like a hot potato.
Not sure of the rates at WL for your stay, but because they are lower than the Poly, it would take a bigger discount to beat the FD.
Hope that helps a little. It helped me to think it through a bit to figure out what I should be looking out for!
