How to handle two houses, one Santa, and a child who notices everything.

Now, let me tell you something. This is one of the kindest things I get asked about, because the people asking are almost always trying very hard to protect somebody small.
If that is you, you are already doing the important part.
It is rarely about presents. It is how will Santa know where I am?
Behind that is a bigger worry: that being in two places means belonging fully in neither. Answer the practical question warmly and you answer the emotional one at the same time.
The answer is simple. Santa goes to where the child is sleeping on the night. If they are at Dad's, that is the address. He has always worked this way, and children accept it immediately.
Some families try hard to make both days identical. In my experience that works less well than making them deliberately different.
If both homes attempt the same big Christmas morning, the child ends up comparing them, and one will always come second. If each home has its own distinct traditions, there is nothing to compare. One house does the big breakfast, the other does the film and the walk. Both are complete, neither is a copy.
The gift competition. It is enormously tempting, particularly for the parent with less time, and children see it immediately.
What a child takes from two parents outspending each other is not that they are loved twice as much. It is that Christmas is a contest they are somehow in the middle of.
If you can manage one conversation with your co-parent, make it this one: agree roughly what Santa brings, and agree that big items come from one house only.
Whatever you have told your child about Santa, both houses need to tell it the same way. A child who gets one version at one house and a different one at the other will work things out much faster, and will feel slightly deceived by both.
This is worth a short, unsentimental message in November. Which gifts are from Santa, which are from parents, and what the rules are.
If your child moves between homes on Christmas Day itself, protect the transition. Do not schedule it in the middle of the excitement, do not make them say goodbye to a half-opened present, and try very hard not to be visibly sad in front of them at the door.
Children take their emotional cue from the adult. A cheerful handover produces a cheerful child, almost regardless of what either adult is actually feeling.
Some years the rota means you are without them on Christmas Day, and that is genuinely hard.
What helps is having your own day rather than a diminished version of theirs. Move your celebration to when you have them and make it fully yours. Children adapt to two Christmases far more easily than adults expect, provided neither is framed as the consolation.
One thing that helps children in two homes is something that is theirs rather than either house's. A message from Santa, saved on a device they carry between homes, belongs to them wherever they are sleeping that week.
Santa can also mention both homes by name, which quietly tells a child that the arrangement is normal and known about.
Arrange a message from Santa that travels with them.
Santa goes where the child sleeps. Make the two houses different rather than competing. Agree the story. Protect the handover.
And be gentle with yourself, dear. Children remember how Christmas felt far more than where they were standing.