Not in any 'ancient Scots' alphabet as such (where did Child comment on it? I don't recall seeing that); perhaps what you remember is a comment to the effect that when Old English adopted the Latin alphabet for writing, it used (mostly) the Irish forms of the Latin letters, which were conveniently available from religious centres in Northumbria and, as time went on, from missionaries at all good shops.

The character in question is 'yogh', a form of the 'g' symbol. I (and others) have explained it in other discussions here, but it resembled a tailed z. As an initial spirant it was pronounced more or less as 'y'; as a medial or final spirant it represented a sort of gutteral sound approaching 'gh'; in glossaries of medieval English it is usually placed (alphabetically speaking) between g and h.

The character gradually fell out of use -rather later in Scotland than in England- and was usually substituted with z (sometimes y) in print transcriptions from older sources. In later Middle English it gradually lost its phonetic value in orthography. It is a very underprivileged character, having, so far as I can tell, no html code to represent it. This seems rather unfair, besides being a nuisance for medievalists with websites.