Welcome to cruise-chat, and congratulations on your upcoming cruise! We took a British Isles cruise last July, although on a different ship and some minor differences in ports of call. In any case, it was mostly smooth sailing, other than some rough waters near the northeastern tip of Scotland and the southeastern area of Ireland, when we were most exposed to open waters. As an veteran cruiser, you know that there are many variables involved, so you might not have the same experience. All else being equal, I'd expect the roughest waters on your itinerary to be between Guernsey and Cobh, and as you round northern Scotland (where the Atlantic meets the North Sea). All this is relative, though -- we've had far worse seas enroute to Bermuda and in the Caribbean.
You may want to bring something for the kids... yet, allow me to suggest another, easy remedy: green apples. They're readily available on the ship, free, and they have no side effects.