So prepare ahead of time and make sure that you've got enough distractions for the kids so that the trip will be uneventful in terms of fighting. Siblings don't usually like sharing stuff so you'll need at least two of everything. Two sets of crayons, two sets of coloring books. And it's probably best to make sure that both sets are exactly the same so that they don't get into an argument over the perception that Billy's coloring book is better than Casey's.
So, what is it that makes fairy coloring such a worthwhile pursuit? This is my opinion, obviously, but here goes. When a child, or an adult faces a crisp white coloring page it is the beginning of an adventure. It gives a little feeling of elation and wonder. It might give you a little feeling of fear or confusion. Trust me, if you relax and realize that you are doing this "just for the fun of it". This cliche is true. Any art is really for the fun of it. For the exploration and adventure.
Learning the color of a chair, or an elephant, of flowers, sun, zebra, parrot etc, will not need mugging up, but mere practical activity that will leave an everlasting impression on these little developing minds. You can also engage them in activities like creating new colors by mixing 2 or 3 colors. This will make them understand the evolution of different colors, which is fun too..
Add all in the list of those, who are participating in the birthday party to make poster board sea creatures to hang as well; starfish, shells, octopus, crabs etc. Inside parties will hang streamers and curling ribbon from the ceiling and pictures on the walls and windows. Invite your guests in with a Sponge-Bob Square pants personalized birthday banner.
When I was a kid I just loved Superheroes and cartoons. It's no surprise then that my favourite thing to do was to watch Spiderman Cartoons! I would plan my whole day around them to make sure I didn't miss them, often running home from school just to see them, even if I knew they were repeats.
In the most basic sense, the act of applying colored media to intricate line drawings is a benefit to relaxation and stress reduction. You are able to put the outside world aside for the moment and focus on the art of coloring singularly.