We tried to double up on excursions on our cruises before, but always end up feeling rushed. We dropped that idea for the future. Now, we just plan one excursion and then use the rest of the time to explore on our own. It is more relaxing and ends up being a lot more fun.
When we rushed to fit in two excursions, we felt stressed at the end of the day. With a single excursion, we go back onboard in the evening, enjoy supper, and have a great time. Since you have two children with you, I recommend slowing down. Some people say this is a once in a lifetime experience, but that is only true if you choose it is.