We cruised with our 3 yr. old grandson's parents on 5 day last holiday season. We left him a bag with his other grandparents that had a wrapped Hot Wheel car and a Little Debbie Christmas tree (his all time favorite snack)for each day we were gone. He opened them every evening after supper time. Then he had a big Hot Wheel to open on the night before we came home. That way he knew when he got his bigger package, mom & dad would be home the next day.
We left the standard info with them ... insurance papers, doctor name and number, shot records, copy of our cruise paperwork with cabin & ship information, toll free numbers for Carnival and then had his 2 aunts call him each evening to talk to him.
By the way, he did just fine and no problems were encountered.