I think it depends on how many days the children are in the program (not day care). We have taken our grand daughters on four cruises and have tipped from $30 to $50 for the entire group of attendants. If they provide dinner for the kids and entertain them in the evening they deserve a lot. Most kids enjoy the programs and it gives parents or grand parents a lot of time on their own. If you feel they enjoyed themselves, tip accorrdingly. Enjoy your cruise with the kids, they will never forget it.
Actually, I have been told that the Kid's Club attendants are considered in the category of ship's officers and are not normally tipped. That's why you don't see it mentioned or see envelopes for it. Some Australian teachers that performed this function stated that they were well paid. One doesn't tip the captain or crew.
However, If someone goes out of their way and does something special, you may want to tip them. Something useful might be a phone card.
If you have young children, 5 and under, I believe that the youth counselors can make or break the whole kids club experience. so knowing that my kids have had a great time I always tip and i agree with frquent traveler around 30 to 50 for the group. Have a great cruise!