A kids' table usually fails for the same three reasons, in nearly every house that has ever set one up.
A kids' table set up out of sight of the adults stops being a table and starts being a holding pen. Same room, in view, is the single change that fixes the most complaints.
There is always a gap between being seated and being served. A name card to colour, a placemat to finish, one small task — anything that fills the gap stops it filling itself with something worse.
Adults can tolerate a fifteen-minute wait for warm food. A six-year-old cannot, and a plate that went cold while the grown-ups were served first is the actual reason the meal ends in a complaint.
There is no fixed age — some ten-year-olds prefer the kids' table for years, and some seven-year-olds ask to move up early. Let the child decide when they ask.
They can, if the task you hand out scales — a colouring sheet for the younger one, a word search for the older one, at the same table.
Part of our guide to Holiday hosting with kids.