Kids you have to Love them ! If it was mine I would tear it down. I guess it ups to you and the age of the unit . The sawdust can travel anywhere and plug up a oil circuit easily . The gravel or sand etc. will migrate to the bottom sump area and can more easily be purged from the crankcase. Fortunate that you eyed the missing oil cap and evidence of tappering prior to starting the unit . If you have faith in oil pickup screens you could flush and refill and see what develops , but again I would tear down to see what actually is down there ?

I recommend locking gas caps for your vehicles.
And maybe a lock for your shed door.
And maybe cut a switch from out behind the barn,
as that would probably be your best preventative
measure.

Is a side cover to the crankcase that would be worth pulling w/o much trouble? Otherwise, it sounds like you will be gambling and possibly a loss of some equipment vs a few hrs. loger