Good plan on locking up the spray paint! When my son(who's 18 now but still makes messes) was about 5, he decided one day that the little blue plastic trays his hotwheels cars went in would look better if they were black.
He found a can of my black spray paint and proceeded to do a good job painting those trays...on the floor of our enclosed back porch, naturally with no paper underneath.
That done, he then practiced his writing skills by painting his name in large black letters on the white door that went into the kitchen...spelled it right too, and it's still on the door.....
Oh, and also, our big yellow dog, who was in the porch with him "helping", wound up with a nice covering of black "leopard spots"....those took awhile to wear off.