Call him on it without expecting him to change his mind. That way, you say your piece without further hurt. Whereas if you say something hoping he'll change plans back to a family day you risk being hurt again when he doesn't.
Plan a great day with your kids.
I wouldn't invite your mum if you know she'll stay with your dad - again you risk more hurt hearing another rejection.
Don't say - the kids will be hurt. Tell him YOU are hurt or disappointed or whatever word you want to use.
It's sad but i would perhaps think twice before prioritising your dad in future.