Snipped for focus....
I betcha they saw all those signs every few feet at Yellowstone referencing the presence of dangerous animals. You know, the bison, wolves, grizzlies, bunnies. As in, "don't go close, don't feed them, stay on the pathway, put trash in bear-proof containers, since if you don't they're guaranteed to get in the trash and then forever after will be associated with humans, and almost certainly will want to eat you up, LVD, AxC, and CD, the moment you're not expecting it. Lunch. EZPZ. Suddenly and out of the blue, no matter where you go, but especially even if you are merely thinking about stepping off the trail. They will know. That bison has his eyeballs fixed on you already. He knows what you're up to. He will get in your face, impale you on those evil-horn thingies, and stomp you to death with those monster hooves. See those wolves over there? Don't turn your back, 'cos they'll tear you to bits and then run off with your arms hanging from their bloody jaws. And don't forget, grizzlies are very protective of their young. They will kill to protect them. Even a drop of blood on those kids of yours, they'll smell it, hunt you down, gouge your face, throttle you with their 2 feet-long claws, and turn you into mincemeat."
These are people who are so afraid of a raccoon, they'll get out a gun and shoot it, no matter it wasn't bothering anything on their property, just making its way along the fence.
So, yeah, not a chance they'd have survived those wildlife protocol signs in Yellowstone.
I agree Plan A was to do something in Yellowstone—maybe even to both of the kids—but they got spooked. I really can't see them giving Tylee a trip as a "birthday treat" from the goodness of their hearts. LVD had no space for that kind of treat-giving or even having a heart.
I have written the above in speculative mode, and I am not quoting Yellowstone signs exactly.