I'll be one more person to chime in on the value (or not) of the DDP. We have been 7 times now in recent years (read "since the inception of the DDP"). Starting with our first, and what DH thought would be our
only, WDW trip

, I priced out the DDP against every menu for every TS ADR we had as well as the CS we expected to visit in each park. No seriously, I have over 25 word doc's and several spreadsheets from our 2007 trip alone, although all are not revolving around dining. I considered the fact that we rarely eat dessert, that in 2007 it included an appetizer
and dessert I believe, and that it was way more food than we'd ever consume. And then ride rides.

Now, the real reasoning I started avoiding the DDP for was the time factor. While we did go to all those TS, and we change it up each trip- maybe some TS, few TS or few TS-- we know that TS meals slow up our ride opportunities. With what we pay to go to the parks, we don't want to feel obligated to use the TS because it's prepaid. Prior to needing to guarantee an ADR with a CC, we could cancel on the fly if it didn't suit. Even now, I can change my plans the day before. Long story short, year 1 we chose not to do DDP and charged
every single item to the room. (And that's when you look and think, did we really eat so often?? So much stuff??) After we got home I went over the room charge printout/bill and analyzed every receipt (I'm sorry if I sound uptight because I'm not, I am just
such a numbers junkie ) and discovered that we saved about $200 over the course of the 10 day trip. But what we actually got included all tips, a couple of souvenir popcorn buckets and drink cups and ALL our tips. So the reality was we saved much more.
Since then, on each subsequent trip we charged every single item to the room and everyone orders what they wish. In the end we actually get less food than the DDP would offer for most meals, but we are never hungry and have snacks and buy waters, etc. in the parks. On top of that we never have to worry about what is included in the meals when doing CS. And yes, being the numbers nerd I am I have done an analysis each time of what we spent vs what DDP would cost. Each year we have saved at least 10%, and even nearly 20%, over the cost of the DDP and that is not considering the beer or two a day for DH and the tips that are included in the room charge. I am always amazed to be honest.
As with everything, it is entirely up to the user and all we can do is let others know what our experience was. But for the OP, I think she may come out ahead of the game by choosing the restaurants wisely after viewing the menus. I also love the idea someone had of letting each girl chose 1 restaurant, or 4 for them total, because perhaps group effort there is smart so that everyone has at least one item on the menu they'd order. Charging to the room is easier than using the DDP, it simply isn't paid up front. With the exception of the MIL who would choose the cheapest item, and I completely respect both parties in that scenario,
just acknowledge to your guests/kids/grandkids that WDW dining is pricey, so get what you enjoy. I'd rather my DD eat and love a $30 steak that hate a $12 steamed burger because it's cheaper.
OP, you are an awesome Grandma to do that for your girls! Have a wonderful time and above all, enjoy!