Prepare for damp weather, and use layers. A stoller or baby wear if that's your thing, is recommended since the usual ports, skagway and ketchikan are literally right off the ship. Juneau is a bit further from the pier. Luckily since it is Alaska, you should be able to easily find any items you run out of or forgot behind.
Must brings for us included some toys and activities, that we would rotate. Kids love to explore and do things, so unless you have access to a kids club (even on Disney open house time was very limited for the nursery) it was a great relief to have our own things to play with.