I have the Morris commentary mentioned. On page 7 he begins
his introdiction to it with:
I like the comparison of John's Gospel to a pool in whcih a child
may .... and an elephant can swim.
He does not say where the quote originated but he is probably
quoteable for what he does say. It may well be that, ike you, there
were so many claimants to the saying that the true originator is
lost in history.
When I wrote a quick not I left that one word out, I guess it is
splash or wade or play.