Why not call the Lagunamar concierege desk since nobody from TUG has responded in years? The concierge desk will at least give you the fee schedule - or let you know it's free.
I own 3 weeks at Lagunamar. I was last at Lagunamar earlier this year. I'll be there for 2 weeks in January, 2012.
I've seen the Kids Club at Lagunamar. It looks just like the pictures. The kids there seemed to be engaged, happy and well-tended. But, I don't usually travel with kids so I don't really know what goes on at the Kids Club.
Lagunamar's a pretty cool resort with the Kids Club and a waterslide in the pool. It also has a McDonald's, a Haagen-Daaz, a Planet Hollywood, a Chile's and an Aquarium (and 2 Starbucks and various other restaurants and shops for adults) in the La Isla Shopping Center right across the street from Lagunamar.
My only caution would be: do not let your kids swim unattended in the surf. It's pretty strong with a rip tide (but that's the way the surf is all over Cancun). ... eom