Busy ECE Mommy - for situations like that I always have extra cards on hand. That way you could quickly write out cards for the child in the morning when you realize that some were forgotten. Then she could have handed out your cards to her friends.
I usually send out an email a week before Valentine's with a list of the kid's names so they have the correct spelling for the cards.
I have to say that I am really impressed with my parents this year. I only had 1 mom just bring cards (that is great too) but everyone else put other stuff with it. There are stickers, tattoos, chocolates, suckers, glow sticks, other treats! I had all kids wear red or pink today (I reminded at pickup last night).
That being said my daughter's pre-school had no parents offer to bring treats yesterday at their party. I was the only one who did (they sent out a newsletter asking for parents to volunteer to bring snacks). I just can't believe that parents can be so lazy sometimes. You don't have to make it, you can buy something if you are in a time crunch.