Are you sure you don't want to host it at your apartment? If there are only a few kids & you said you don't have a huge family & list of friends, right?
I would clear out the main living area and cover the walls and tables with plain white paper, that you can buy by the spool and/or a couple plain white topvsheets and hang them up and let the kids go crazy with markers/washable paints...wouldn't that be cool?
I envision the kids walking into a room that is completely covered (walls & floors) with all white sheets, helium latex balloons w/the strings cut off so they were all over covering the whole ceiling....and then you just cover them with smocks, have them take off their shoes and direct them towards the pile of art supplies!!! Brilliant!!!
*You could put something on the invitation about wearing clothes you don't mind getting dirty and how they are invited to a very 'colorful celebration' or something to that effect? How exciting would that be?
You could cut out holes in black trash bags for make-shift smocks and literally let them go 'Art Crazy'!! As long as you covered the floor and everything that you couldn't move out of the way with something(thrift store sheets or a tarp or something)I don't even think it would be all that messy? At 9y/o most kids follow directions pretty good. I bet it would be so much fun, be relatively inexpensive to pull off, very unique & then your son would have a great mommento from his special day, party favors would be easy b/c you could let them take home whatever section they worked on, ya know?