First, I don't quite agree with this statement:
great is the only common English word in which "ea" is pronounced /eɪ/.
Break and steak are pretty common, and both have the /eɪ/ sound.
That aside, this goes back to the Great Vowel Shift, which is the cause of many of the peculiarities of English spelling. The linked Wikipedia article gives plenty of information, but the short version is that while most words with "ea" shifted to the /i/ sound, as in beak, some didn't, possibly because of the influence of the consonant following "ea". Great, break, and steak all have plosive consonants after the vowel; the "r" in bear and swear pulls the sound of "ea" in those words a different way.
And yes, I know, beak also has a plosive consonant, and fear ends in an "r". Changes in natural language are rarely consistent or easily explained, and this is one case where we just have to accept the fact that some words changed their pronunciation in a certain way and others, for whatever reasons, either stayed the same or changed in other ways. In other words, to quote Seth Lerer in his lectures on the History of English (2008):
As a coda to this lecture, let me mention a small group of words that seem not to have undergone the GVS. There are a small group of words that are spelled with -ea-, steak, great, break, and if these had participated in the GVS they would have been steek, greet, breek, and this is not something that affects every single word with an -ea-, it is not something which affects lots and lots of names spelled that way. But etymologically and historically, the words steak, great, break, should have participated in the GVS, and been pronounced steek, greet, breek. Why this is, nobody knows. So now I'm going to leave you with this provocation that even though linguists may think they can explain everything, there are gaps in our knowledge and exceptions to our rules.
William Safire New York Times, "On Language", March 13, 2005 traces first usage to a 1984 Biography of Barbara Hutton (evidently Poor little rich girl: the life and legend of Barbara Hutton by Heymann):
''delighted but not surprised by the enthusiastic 'money quotes' in
early reviews.''
Although since the author puts it in quotes, the sense must have been in some use in 1984 already.
He traces this particular adjectival sense of 'money' back to 1890, from 'money players' through 'money position' to, yes, the 'money shot' in both pornographic and photographic senses.
I will let you use your own judgment on Safire's own reliability.
EDIT:
I took a few minutes to search through various corpora at http://corpus.byu.edu/ and came up pretty empty. COHA had nothing, BNC had nothing. The Time Magazine Corpus had its earliest use in 1990.
Their interface to Google Books found a valid reference in 1986 in Rock of Ages: The Rolling Stone History of Rock & Roll.
So far it looks like Safire may be right (!) and 1984 is about the right time period for first use. Seems kind of late to me, but intuitions about phrase origins are often wrong.
Best Answer
Explaining to a 5-year-old
English spelling is quirky. English spelling tends to be more influenced by how people spelled words over a thousand years ago than by how we pronounce the word today. Yes, that means that word pronunciation has changed drastically over time, and our language today is not the same as the language that was spoken back then.
One of those changes is that the /k/ sound, written using the letter C, came to be pronounced /s/ in some cases. That change probably happened back when people were still speaking a language called Latin (which nobody speaks anymore). The Latin word was circulus, and in the early days, both c's were probably pronounced as /k/. Then people found it too awkward to say "kairkooloos" and started pronouncing that word more like "sairkooloos", which eventually became our modern word circle.
For adults
The /k/ sound is one of the consonants that has a tendency to mutate over time, and that mutation even has a name: assibilation.
Originally, Latin had a letter C that always represented the /k/ sound. In the later days of Latin, the letter C would sometimes represent an /s/ sound when placed in front of some vowels. Confusingly, even though Latin was a "centum" language (meaning that it fell into the /k/ category in the k/s divide), its word for 100 eventually came to be pronounced with an /s/.
I believe that the word circle follows a similar history: originally pronounced in Latin with /k/ sounds, and later the first c came to be pronounced with an /s/.