Practical adjustments for children who find surprises, crowds and abstract stories difficult.

Right, so. I run the systems side of the North Pole, which mostly means I spend my time making complicated things predictable. That turns out to be exactly what this topic needs.
Christmas is loud, unpredictable, socially demanding and full of abstract concepts presented as fact. For a lot of children, that is a difficult combination. Here is what actually helps.
There is no single adjustment, because the difficulty differs enormously between children. Work out which part is the problem before changing anything.
Each needs a different fix. Solving the wrong one is why some families try very hard and get nowhere.
Mall grottos are close to a worst case: unpredictable queue length, bright lights, noise, and a stranger who may reach out. Many families skip them entirely and lose nothing.
What works better is a controlled environment. A video call from your own front room, at a time you choose, with the volume where you want it and the option to end at any moment, removes almost every sensory variable at once.
Some children will watch from the other side of the room the first time. That is a completely successful first attempt.
The expectation to talk to Santa, smile and answer questions is a great deal to ask. Take the pressure off explicitly and in advance.
Tell your child they do not have to speak at all. Tell them you will do the talking. Children who are told they can stay silent very often speak, and children who are told they must speak very often cannot.
This is the part parents find hardest, and I would encourage you not to fudge it.
A child who thinks precisely will notice that a man cannot visit every house in one night, and telling them it is magic may not satisfy them at all. Some children are genuinely distressed by being asked to believe something that does not add up.
Two approaches work. The first is detail: give them real mechanics, a route, a timeline, an explanation with numbers in it. Precise children are often satisfied by precision rather than by mystery.
The second is framing it honestly as a game everybody plays. A surprising number of literal children love Santa more once they are told it is a story people do together, because it removes the pressure of having to believe something they cannot.
Neither is a failure. You are not depriving them of magic. You are giving them a version they can actually hold.
Predictability is the whole treatment here. Tell them what will happen, in order, and then do exactly that.
Christmas is often built around surprise, and surprise is exactly what many of these children find hardest. You can remove nearly all of the surprise and keep nearly all of the delight. Those are not the same thing, even though we tend to bundle them together.
A child who knows precisely what is going to happen can still be thrilled that it is happening.
If a grotto is not workable for your family, a private call from home lets you control the timing, the volume, the length and the exit. You can also prepare your child by showing them exactly what it will look like beforehand.
See how a call from home works and decide if it suits your child.
There is no correct way to do Santa. The version that works for your child is the right one, whether that is full belief, a shared story, or skipping the parts that hurt.
Awesomeness stats do not require a grotto photograph. Trust me on this one.