Thanks to all who replied relating specifically to the packaged foods and for the list of cereals they DO have.
Keggy, I'm on your sailing, but thanks for the offer to check for me.
My son has a lot of food allergies, so simply relying on what's available in the ports isn't a great option for us, especially if we can't find highly fluent English-speaking wait staff. I'm doing my best to research all of my options in advance so we know if we have to make arrangements to return to the ship for lunch, or if we can stay in port all day. Yogurt and uncrustables won't work for him, but some cereals and chips will.
We do plan to get fruit as well, but I have been told we probably can't take it off the ship at most places, so I don't want to rely on it. I'd heard we can't take sandwiches off either (and we can't know in advance if he can even eat the bread they have). And as for getting it in the ports, I do worry about it upsetting their stomachs. They'll be eating plenty of fruit on board, so I don't worry about giving them cereal or other dry snacks when we're off the ship.
Thanks again for letting me know what's available.