From when my children were born we always said all presents were from Santa - even presents from family were just put in with what Santa had bought. We said this to all family and they were obviously under no obligation to buy for the children as they wouldn't get any credit for it.
Most family were fine with it (or at least didn't say anything to my face!).
Kids are now 7 and 8 and both still believe.
My Dad has messaged this year saying oh we are going to put messages on as we want them to know who they are from. I have gone back to him and said that he should save them for the kids birthdays if he wants them to be from him. Both birthdays Jan and Feb anyway.
AIBU? I just don't really know how I can change the whole gifting from Santa now. I maybe should have thought more about it when they were younger.