I agree that a pond and twin boys would be dicey.
(My twins are 19 years old ;-)

You might call around to various stores that sell fish
and see if they know of a pond/koi club. When I had
to reduce my stock the local club publized it and came
out to help with the capture of the fish.

The waterfall/ rock feature sounds wonderful but... keep
in mind that you are going to need to get to the pump
from time to time. Dust and debris will drift in and could
clog the pump. Or the pump could give up the ghost.
Plan for a way to remove all that rock. You might consider
having a grid built of rebar so you only have to pile a
small bit of rock on top.

Setting the pond back up again when your children are older
is a lot of fun. I have great pictures of my youngest
and dog going frog hunting. Frogs 99, boy and dog 1.

kathy :-)