I'd suggest keeping it simple with a basic art kit that has different colored construction papers, safety scissors, glue sticks, and maybe some sparkly stickers or sequins for some bling! pipe cleaners, pom-poms, and Googly eyes are a fun addition too. That should be plenty to fuel their imagination and encourage creativity.
For a bit of variety, you could also keep different textured materials like straws, strings, and cardboard tubes - great for some 3D craft action! And of course, don't forget the paint! Face paints or poster colors are fun for little artists. Just make sure to supervise them during the artsy mayhem!