While that may be true depending on cabin location in relation to the parents if they call Royal to remove the cousin ahead of time but if the cousin is a no show at the pier then they will be fine as once on board no one cares and worst case scenario is the child sleeps on the couch
Also, there are several travel agents that post on CC and a whole lot of posters that know a whole lot more than many of the travel agents out there so asking here is a great place to turn for advice.