Dear Ewan:
It isn't "Camptown Races", it's "Polly Wolly Doodle", one stanza of which goes [or did until it was seen as racist!]
"Oh I came to a river, and I couldn't get across/Sing Polly etc./
So I jumped on a nigger, 'cause I thought he was a hoss, /Sing Polly etc."

This I have found in a version of your song [what is it, actually?] namely "Hey, Cocky Doo", from Dundee, in the Montgomeries' "Sandy Candy", 1948:

Hey, Cocky doo,
How d'ye do?
Sailin aboot
In yer best o blue.

An alpaca frock,
A green silk shawl,
A white straw bonnet,
And a pink parasol.

I gaed tae the river,
I cudna get across,
I paid ten shillins
For an auld blin horse.

I jumped on his back,
He fell wi a crack,
So I played the fiddle
Till the boat cam back.

The other song is probably not connected, for I'd say the names/rhymes are coincidental. I'm reminded though of a bit of a version of "Sally Water" (as the English collector Lady Gomme titles it) in her "Traditional Games of England, Scotland & Ireland", II.163, from Aberdeen Training College: "...Raffles up, and raffles down, and raffles a' a dancin',/ The bonniest lassie that ever I saw,/ Was [child in the centre] dancin'." This itself reminds me (albeit faintly) of the 5th verse of Robert Burns' version of "The Ploughman": "Snaw-white stockins on his legs,/ And siller buckles glancin;/ A gude blue bannet on his head,/ And O but he was handsome!"

I'd appreciate a word from you (by private e-mail if you like) about your rhymes. Cheers
Murray@saltspring.com