There are 10-12 counsellors in each of the kid age groups. Since that gets to be a large (and expensive) amount, I do what you planned. The kid navigator has photos of the counsellors. I ask my DD (privately in the cabin) if there were one or two who made her time special. Then we put the gift card in an envelope with a thank you note adressed to that person.
My understanding is that if you just give the cards to the club or lab, they are divide at a party that the cast has periodically. If you want your gift to go to a specific person, adress it to that person. That way, even if they are off on the last night, they will get your gift.
Since your 4 and 7 year olds will both be in the Club, I think 2 is plenty. We do one or 2 from each child. Realize that this is not a normal "tipping" position--the gift is nice, but it is not expected. That's totally different from the servers and room stewards who basically work for the tips.