Palatalisation is the technical term for what caused the change from /k/ to /t͡ʃ/.
You might have noticed that the vowel in the word keep (/i/) is produced further forward in the mouth than the vowel in cool (/u/). The KEEP vowel is therefore called a ‘front vowel’ and the COOL vowel is called a ‘back vowel’.
The /k/ in cool is articulated in the back of the mouth, meaning the closure is made further back in the mouth whereas the /k/ in keep is articulated further forward in the mouth in anticipation of the following front vowel—/i/. Because of the following front vowel /i/ in keep, the closure for /k/ is made further forward in the mouth (closer to the palate) in order to ease the transition.
In Proto-Germanic (ancestors of English), most of the /t͡ʃ/-words were pronounced with a /k/, for instance, child, chin, cheese etc., were pronounced with /k/ (they're still pronounced with /k/ in German). The word church was *kirika in Proto-Germanic (compare Scot. kirk, from Old Norse which retained the original /k/). [Trask Historical Linguistics]
So what happened?
Over time this palatalisation of /k/ before a following front vowel went so far that the /k/ became [tʃ]. That is to say, the closure for the /k/ moved to the front part of the mouth, resulting in an affricate [tʃ]. [Trask Historical Linguistics]
An example of 'palatalisation' in Modern English is the pronunciation of what you as whatcha: the /t/ in what is alveolar while the /j/ in you is palatal, the /t/ in this case is articulated near the palatal in anticipation of the following /j/, resulting in an affricate [tʃ].
According to Peter S. Baker (Introduction to Old English), Old English c (which is sometimes printed with a dot: ċ) was pronounced /t͡ʃ/ before the front vowels i and ie and the diphthongs ea and eo.
I don't know what happened to kettle, but here's what Donka Minkova has to say:
The PDE vocabulary shows many such pairs. The presence or absence of palatalisation of an etymological [k] to [tʃ] is responsible for the different initial consonants in cold-chill, kettle-chettel (dial.). Alternative pronunciations in a derivational set or in OE and ON are also behind the [k] ~ [tʃ] alternation in the histories of bench-bank, birch-birk, chest-kist [...]
[A Historical Phonology of English by Donka Minkova]