A mate did it both ways with his then 7-year-old lad in 2012.http://sometimesridesbikes.blogspot.com ... st-to.html
The boy (Tom) went on to become the youngest person to do:
the Festive 500 (pretty much the first few years he did it he got an honorary mention as "the youngest rider") https://minipips.blogspot.com/2012/12/r ... 8-wet.html
youngest person ever to do an Everesting: https://minipips.blogspot.com/2017/09/e ... n-dam.html
plus a few other things like 24hr racing, the Tour Divide
I doubt he's the youngest person ever to do C2C but as mentioned there are several versions of it and several ways of determining "doing it" - is it in a day, over several days (and if so bikepacking or B&B?), does doing it on a tag-along or trailer bike or child seat count? They just did it as a fun trip back then, there was no interest in setting any record anywhere.
*edit: actually at the bottom of the page it says:
Postscript - The definitive Hadrian's Cycleway Information Trip Section now says
"Bowness to Tynemouth and back again.
Father, Richard and son Tom age 7, recently completed the 200 mile round trip in late October.
WELL DONE - Tom is the youngest to achieve this".