That's what I do. If I see a room that fits my family of 6 and I like the price, I book it for the first 4 people, and then I call in to add my other 2 kids to the bookings. You can get a good idea of the additional pricing by looking at the 3rd/4th pax pricing - it's going to be the same. The reason why I book immediately is the cabins for 5+ are very hard to find, and I don't want to waste time waiting on hold for a rep. I've never had an issue booking this way. Also, when you add the 5th/6th passengers, Royal will create a separate booking number for them - each reservation can have a max 4 people listed on it.