My PIL and SIL live on the coast, an hour away from us. We try to see them every two months, and will usually celebrate birthdays on the closest day possible. So my DD and my niece both have Nov birthdays. We saw them in Oct, when we celebrated mine and DH's. They came to us this Sunday, for an early Christmas lunch (we can't see each other over Christmas) and we gave our niece her bday gift. Nothing was said about my DD's, and at the end of the day my DD commented that we'd given her cousin something, but she'd not got anything.
They are well off and usually spoil the kids, so I'm guessing they genuinely just forgot. So, do I ask them/remind them, and risk embarrassing them and myself or do I suck it up and keep quiet, even though my DD was upset? For myself, I'd be mortified if I ever forgot a close relative's birthday, and would prefer a reminder than to have them keep quiet, but I am super emotional and hate for anyone to feel left out.