I would never to expect any child, under the age of 10, to endure a 1.5 to 2 hour formal dinner without some sort of distraction. Especially if there were a majority of adults at the table with adult conversation. I also believe children should be exposed to formal dining as often as possible. They will never appreciate it or learn the protocol without being exposed to it. Color crayons and a couple of coloring pages would be fine for small children. I would take Trip's advice and bring your own crayons and a couple of pages out of a coloring book for the children.
I commend you on changing your table to a four top. If you are ever on my cruise you and your children are more than welcome to dine with my wife and I.
Heck, sometimes I can't handle a two hour dinner if there is boring conversation.