I give my adult children cash for Christmas. Always the same amount. Two are now married, what should I give their spouses?
Is this a question of ethics or financial (where money might be too tight for you to give all parties involved ... including spouses)?
X = the amount of cash you give each individual child.
If you can afford it and are comfortable:
Give each individual the X amount of cash or
give each couple X+ (all the way up to 2x).
Take the cash and buy a couple's gift.
If money is an issue:
Just let them know that you don't feel comfortable with continuing the past tradition. Evolve the yearly gift of cash into ...
like Jespah said... an affordable dinner for you and the couple; etc....