Most homes will have coloring pencils or crayons. In my house we also have glitter pens, foam pens and most importantly paints for the kids to select between. Using paints will require a little bit more setup time as you may need to clear an area and lay newspapers down over the table to protect it.
Don't limit what they can offer due to the fact that they don't have many supplies to work with. You can even consider offering them craft supplies such as glue and glitter, cotton, ribbon, and other items that can all become a part of the Spongebob coloring pages they are completing.
This is the first place your kids can have fun. By letting them choose their pictures they will enjoy having the choice. Children like to feel they are big enough to make decisions and that their desires have an impact on the world, so let them browse the available pictures until they are happy.