We like the ease of the dining plans and feel they are a good value. However, we are not interested in using our villa to eat, either, other than for snacks and drinks.
In your case, I would say you would have to look at what the DDP gets you beyond the TS meal (which you are doing anyway), and whether or not you plan on using those pieces.
If you plan on eating every lunch in your room and doing no CS meals, then it really doesn't make sense to go with the DDP. However, if it is only going to cost you a little bit more than what you would pay for your TS meal anyway, and you end up with a CS meal and a snack for everyone, then it might be.
For example, we are going to Le Cellier and I plan on getting the Chef's Favorite. This, plus a soda, with tax will cost me $42.00. That is the price of the plan. Basically, that day, my CS lunch and snack are free. For my family, what we would eat at the restaurants if not on the plan provides enough value to make it worth it for us.
What you may want to do is look at some of the menus of the places you plan to eat and price out what you would eat if not on the plan. So, if you won't order dessert if paying OOP, don't count that in your figure. If you would order an app paying out of pocket, then add that cost to the price of the DDP as those are not covered. Don't worry about tip as you are going to pay that anyway since you already know you are doing 1 TS meal a day.
See where you come out. The fact that you have a child who may choose to eat off the kids menu but you have to pay adult price, may be enough, to negate any value there is.
Good luck!