happy birthday Marlowe 
Time for some tough love - you're being taken for a mug
Whose birthday is next ? If it's the 7yr olds then leave this for the next one. Nothing - no card, no cake, no present, no acknowledgement. Nothing, zilch, zero, nada.
I know it sounds harsh and it is. But their treatment of you is totally unacceptable. If money is tight there's plenty they can do - breakfast in bed, cook all meals and tidy up afterwards, homemade cards etc.
I personally don't sign up to the 'don't give to receive' ethos. I'm much more a 'you scratch my my back, I'll scratch yours'. Reciprocity is the name of the game. Tit for tat in game theory is the strongest negotiating ploy - I'll do something on trust for you, but if you don't play at returning the favour at the 1st opportunity - all bets are off.
And don't you get tempted to be nice for his landmark birthday. And don't remind the kids either for that matter. If he starts hinting, play dumb 'Really dear, I didn't think we were marking birthdays anymore. You didn't bother with mine so .....'
My DP didn't bother for a Mother's Day when DD was very small and too young to remind him. Fine. Claims he's not bothered about birthdays, Christmas, et - yeah right. Come Father's day - from me nothing, zilch, zero, nada. He's remembered ever since. play hard ball girl, play hard ball.