I have used 'Master' to address correspondence to youngsters in the past but I now see little point.
For example, I used to address mail to an adult as to 'Mr J Smith' (or 'J. Smith, Esq') but now I'll probably just put 'John Smith'. Since I no longer use a title for his father, it seems illogical to use one for a child.
Debrett makes no mention of forms of address for children but I note that omitting any form of title (on an envelope) for an adult is now considered to be acceptable:
http://www.debretts.com/forms-address/titles/untitled-men
(It would thus seem to follow that no title is now required when sending correspondence to a child).
Possibly also relevant:
https://en.wikipedia.org/wiki/Master_%28form_of_address%29