it's the age. Mine was sick all the time from the time he started preschool at 4 and until about 2nd grade.
He started cruising when he was 3 and only got sick on a cruise once, and it was our own friends' kid who got him sick (their kid was still sick when they boarded the ship with us).
I do make sure to give him his regular vitamins and probiotics every day on the cruise, and take ALL imaginable meds with us as well. He also never sucked on his fingers.
Oh, the "funny" thing is - we got covid on a cruise twice already, but our kid didn't.