The message
from Chris Bacon contains these words:

The ground's very well drained, fortunately. I was going
to line the bottom of the hole with weed (and hopefully
worm) proof permeable membrane (Terram).

You'll find the kids often like to dig as far as the membrane then poke
holes in it. I've seen them done where the membrane was covered with a
layer of paving bricks. Still plenty of gaps for the water but less
tempting for the kids.

--
Skipweasel
Pay no attention to the man behind the curtain.