Squirrels are rats with good public relations. Buy a few victor rat traps super cheap. Especially if your going to drown them or suffocate them anyway, put em out of misery quickly. Also, you could get one of those fancy new alligator looking rat traps. I put a few of those in my attic, no more squirrels. As far as the possum goes, get a .177 pellet gun. They don't count as a fire arm by federal law. For about 50 bucks a wally world you could get one with a rifled barrel and a scope. That will take care of your possum problem. Depending on the age of the kids they would have a hard time pumping the pellet gun up enough to hurt anything, if they managed to get a hold of it.
the rat traps work well if you tie a chain or string to the trap you can find the trap and use it again if you put a coupla washers or pennies under the spring that rest aginst the trap the trap dispatches quicker , most Localities consider a air gun a firearm and will generally charge you with the same infraction that is covered in the firearms law