Now… this doesn’t have to be difficult… There are some great kits out there that come with instructions. You can even use Stencils… so it’s as easy as coloring in the lines. There are even stamps to use!
You can be make it as elaborate as doing a full Cat or Clown face… or as simple as painting a spider on a cheek.
I would suggest having help with this. I usually hire a teenager (either a sibling, or a babysitter type) to help. Explain to them EXACTLY what is expected… and let them be creative. (You might want to mention that bloody icky stuff is not acceptable…. ) Let them practice with the kit before the party. And pay them for their time (or work out a trade with another mom to help at her next birthday).