There are some excursions that are simply NOT suitable for young children even though they are allowed to go.  I've heard parents say afterwards ...I wish I knew....The White Pass & Yukon Route Railroad  in Alaska is at the top of the list.  It is an awesome train ride reaching 3000+ ft altitude overlooking gorgeous terrain. Often there is only a few feet between the train track and the valley below.  It is a three hour round trip - NO STOPS - and you turn your seats around to come back down the mountain. The aisle is about two feet wide.

A family got on with a 2 and 3 year old and before we even left the station, grandma got off with the screaming 2 year old.  The 3 year old hated the train & cried non-stop.  When we returned with frazzled nerves, grandma and 2 year old were waiting, happy and refreshed after 3 hours of "relaxing" - the 3 year old was a basket case. The parents were worse. (To their credit, they tried everything to soothe the child as did other passengers).  This is not a Thomas the Train experience!