These are in online dictionaries and described as ‘imitative’ and ‘child’s term’ but how far back do they go in English? Who first got in print with one and what was the context? And are there any others? (There is ‘baa-baa black sheep’ but only in the nursery rhyme I think.) Do other languages have equivalents?

The most famous use is maybe Joyce’s opening sentence:

‘Once upon a time and a very good time it was there was a moocow coming down along the road and this moocow that was coming down along the road met a nicens little boy named baby tuckoo ’

Researching this I also came across an idiom ‘Why kick a moo cow?’ which was new to me and wasn’t explained though I take the meaning to be ‘easy target’ stuff.

Waikikamukau is a standing NZ/Australian joke. It is a Maori-sounding name for a non-existent town: it just means a general small town in the middle of nowhere.

The earliest printed “moo-cow” I can find is Isaac Taylor, Scenes in Europe (1818), p. 20:

So moo-cow, and piggy hog, how do ye do,
No danger of tearing, or tossing, or mire :
You rabbits, and hares, I can run fast as you ;
You seem all very cold, will you come to my fire.

For “choo choo train,” a caption in Judge’s Library, Issue 16 (July 1890), p. 7:

Howling Blue-jay (picking up a discarded banana-skin)—” White man must have heap big fight on choo-choo train. Lose lot bald-head scalps.”

An 1810 reference to “moo-cow” from the Poetical Magazine:

The sheep all baa’d, the asses bray’d,
The moo-cows low’d, and Grizzle neigh’d:
“Stop brutes,” he cry’d, “your noisy glee;
“I do not want to hear, but see;
“Tho’ by the picturesquish laws,
“You’re better too with open jaws.”

http://books.google.com/books?id=yQsUAAAAQAAJ&pg=PA54&lpg=PA54&dq="the+moo-cows+low’d"&source=bl&ots=QtKsw_ljeL&sig=YkWGCVs48IKG7u7i3HSu0bnu3FE&hl=en&ei=F7XJTISZBMWOjAerubnEDw&sa=X&oi=book_result&ct=result&resnum=1&ved=0CBIQ6AEwAA#v=onepage&q="the%20moo-cows%20low’d"&f=false

And are there any others?

http://www.youtube.com/watch?v=N-9YStmD5HE

Thanks for these - we can now add bow wow to the meagre corpus. The origin of such words has fallen through the cracks because stuff in wax crayon has never been published, or children’s speech recorded, except when adult writers imitate it even to this day.