We had our 8 yo with us on last years Iceland Cruise. When we overnighted in Reykjavik, we did a 12 hour Golden Circle tour the first day, then left the kids (8,16) on the ship the second day as our excursion left early the second day (they needed sleep, so I planned it that way). Otherwise, our 8 yo had no problems on any of the excursions (
DCL, private), and enjoyed them all. I think a 6 yo could participate in a lot of things on this trip.