For all the free dining, buy 4/get 3 free deals of the past few years we always bought the fewest park ticket days needed (usually 1 day tickets) as we've either had APs or some old non-expire 10 day tickets. We still have many of those 1 day tickets and as a previous poster said, they are valid tickets. We use them as "deposits" on new APs or new 10 day passes - but they are valid 14 days from first use.
I have heard stories where the front desk clerk will ask to see all members of the party at check in before releasing the tickets. They have never done this to me - but I have traveled a dozen times to WDW and always have my DW and DDs with me. When I make a reservation, they know their names and their ages - so they know my history. If I suddenly added a 3th child to a ressie that was not an infant, maybe they would flag the ressie to make sure I really had a 3rd child with me. Not sure.
I see you banner states the trip is to celebrate a new foster child. Congratulations!
That said, if the child is not coming with you, its unethical to add him or her to the reservation.
I find this question similar to the following: A DVC member who has a family of 4 (lets say its a DH, DW, 2 kids over 10 years old) uses their points to get a villa. Since a DVC reservation does not get free dining, they also rent a room at a Value resort. Pay $80 for the room and get $170 worth of free dining (lets assume its the old dining plan where you get the normal dining plan for free). I almost confronted this issue head on as I booked a bounce back free dining at POP (so I am getting the regular free dining under the old rules). I booked it last August and am going in a few weeks. In the interim, I bought DVC on the resale market. I was thinking of splitting my stay between POP and a DVC - but should I keep the POP ressie for the full trip to get the free dining? I decided it would not be ethical. So I decided to save the DVC points for a future trip and stick with the original plans.