T-Rex is slightly surprising, but MK is not.
You can keep trying back closer to your trip. T-Rex is one of the few restaurants at Disney I've repeatedly been able to do without a reservation, though generally I have to wait for 20 - 30 mins, but that's not a problem. There's a small play area called the "Bone Yard" and a "Build-a-Dino" (done by the Build-a-Bear people) gift shop there to keep the kids occupied during the wait, and they give you a pager (as you'd probably expect), so you can just wander around and the time passes by pretty quickly.
One quick note about ADRs...if you do NOT cancel them the day before (24 hours) because you changed your mind, or for whatever reason you fail to show...
You will be charged $10 per person on on the reservation to your credit card.
So, for DTD, if you really had your heart set on T-Rex, you may (note...MAY) want to cancel your Planet Hollywood ADR to give you the flexibility to see if they have any day of seating available.
There's enough at Disney Springs to eat that you will be able to get in SOMEWHERE...the question is where. That said, I've walked up to Rainforest, Planet Hollywood and T-Rex without ADRs in the past, and after a wait, been seated.
Also, if you really want a TS experience the night of Magic Kingdom, try looking at one of the monorail resorts (Contemporary, Polynesian, Grand Floridian). Ohana or Kona Cafe are both great food wise (at Polynesian), or Chef Mickeys(Contemporary) and 1900 Park Fare (Grand Floridian) as those are character meals.
It would be easy to leave the park say, around 6p, hop the monorail to one of those resorts, eat, and return to Magic Kingdom for the evening.
Since you got Memory Maker, I highly recommend you look into character meals for your TS experiences. Especially as you mentioned the 5 year old can be a bit choosy. A lot of the character dining experiences are buffets (Crystal Palace, Chef Mickeys, 1900 Park Fare, Akershus...though, that's only a partial buffet and it has Norwegian food like lox...mmmm....lox, Tusker House, Hollywood and Vine), and with your Memory Maker they pictures they take (when you show up, you'll get to pose for a special picture with the "host" character...so, Mickey for Chef Mickeys, Donald for Tusker House, etc., before being seated.)
Normally those pictures cost extra to purchase (around $25, iirc), but you'll get them for "free" with Memory Maker. A few character meals and some on ride photos, and you've paid for Memory Maker plus some!