About tips for children, and tips in general. Where else, for $3-5 can you feel generous as all get-out. If a tip of $50 is expected and you give $55, you are a big spender. How else could you spread so much joy for so little? While I agree that the cruise lines could pay their staff better, the cruises would then cost more and you would miss out on the opportunity for generosity.
Generosity rules. Enjoy it.