Most Balcony cabins usually have sofa beds and looking at the deck plans, from what I can see it most probably is a sofa bed (Dont think I have ever seen bunks for Balcony Cabins on any ships). I hope your 8 and 17 year old get along as it look like they will be sharing the Sofa Bed. PS Someone else asked a question on cruising with kids on MSC which I answered a couple of days ago. Will give you lots of useful tips pertaining to your 8 year old if you look back 2 or 3 pages.
Go to MSC website/fleet/orchestra/decks. Some are sofa beds, some are bunks, just look for your cabin number & check the symbols against the legend.