hilltopfamily,
I think that is up to you. We gave cards to all the counselors for the 5-7 age group which my daughter was in. This was either 10 or 12 I believe. Of course, I only had one in the clubs. I had cards for the counselors in CG, for older DD, but she did not spend much time there so we did not give those out. We even gave a card to one counselor who was with the 3-4yos. She knew us by name and we spoke to her a lot during the cruise, so since I had extra I gave her one.
One other thing that I did. I made up little post cards with a picture of the girls, date of the cruise and a little thank you message. I put these in a envelope with the gift cards, and that is how I handed them out.
Again, it is entirely up to you who you give the cards to. They are not expected like the other tips. If you can afford it I would give one to all of them, but if it is a cost issue, than just give it to the ones who you think spent the most time with your children. Also, if both kids are in the same age group then one card from the both of them should be fine.
LeiLani