Creating a birthday banner on a thick A0 white paper is a great way to keep the children engaged while you set up for a party as well as to keep the early finishers busy.
Create the poster before the party using bobble letters and fun theme-related pictures so that the kids can colour them in
Place the poster on a table (or on the floor on a plastic cover) away from your crafting table
Add one or two water jugs with brushes and tempera paint blocks so that the children can help themselves
When they arrive at the party they can start painting, then you can do the main craft and then they can continue painting once they are done with the main craft