The Multiguard snail pellets - it's basically a chellated mineral in a sawdust base - no harm to animals & it breaks down to add minerals to the soil. Have been using them for years. Get it down asap, once they find your greens they can decimate them in a night or two, if they're small.

Lenny the tubing idea is a great one.

Re the caterpillars, there is a spray called Dipel which is essentially a microorganism which is death to caterpillars but no problem for bigger animals...quite acceptable in organic circles.