+1 on Gary. I let my show run unattended. You would have to be really careful with a laser - both where it is mounted and where the beams are pointed.
Kids are kids, they do stupid things... Sometimes because they don't know any better, other times on purpose...
Also, be aware that if you are not just drawing pictures on a wall, you will probably need a really big fog machine. To see the laser "beam" you need a lot of small particles in the air for the beam to bounce off of. Smoke, fog, pollen, etc. Outside, the fog will tend to dissipate quickly as the space is not contained...