As for swimming- the snakes don't randomly swim around in the pond. They'd
rather lie near or on the bank and wait for prey to come by. So, if you walk
to the pond with the kids, and visually clear the area of your "beach", the
kids should be able to go in and out in the same general place, and swim
around unmolested. Whichever adult is supervising the swimming can easily
keep the beach snake-free.
I highly recommend ducks. They are hilariously entertaining, low maintenance
once they're beyond ducklings, and could be used for meat or eggs.
Warren
PS- I'd skip the dynamite- it'll kill the fish and spare the snakes.