DS3 was 3 when we went (also with grandparents) but it was a Disney cruise. They had a pool but it was too cold, so we only went once. He went to the kids club but never lasted that long. We took him to play games, watch movies, character meets, etc. He would also still nap a bit. My older two were 7 and almost 10 and they were great. They liked the kids club and my oldest spent most of his time on the top deck on the sports court with the other kids. We planned our excursions around the kids and they loved those.
Mama to my boys (04,07,11)