Happy to help.
Yes, I would try to do the journey with your DS the first time. Neither Southend Victoria nor Shenfield is a complicated station to use, but sometimes the timings are a bit tight. DS1 and I did Chalkwell to UEA yesterday (four trains and a bus going, mercifully only a bus and three trains coming back as we were picked up in Southend) and the booking gave us seven minutes to change at Colchester going out - which could have been a problem - and about twenty seconds coming back...
By the way, your childhood reminds me of a friend at school, whose father decided when she was about 13 to take a two year contract in what was then the Gilbert and Ellis (?Ellice) Islands - postal address, 'via Fiji'
. A Universal Aunt - remember them, anyone? - used to put her on the flight at the London end, but after that, she was on her own.