Bring scissors (in your checked luggage of course). You'll need them to cut loose threads off of clothes, tags off of new clothes that you buy and open packages.
If you are going to send postcards, print off address labels on your computer before you leave and you'll have them to just stick on the postcards.
Tylenol and cold medicine for the kids, just in case. Also for you if you're particular about the brand. Your hotel will sell something, but it may not be what you are used to or what will work for the kids.
Ziploc plastic bags - lots, in different sizes. You can use them to pack wet clothes when it's time to come home and to carry snacks into the parks.