I have also been working of late on media constructions of childhood.
One possible explanation for many of the films from the 1950s that
you discuss centers around the dramatic shift in our conception of
childhood and appropriate childrearing techniques. In the pre-wwII paradigm,
children were assumed to be dominated by base desires and the goal
of parents were to exercise strict authority to prevent them from acting
on urges towards masterbation and sexual exploration. In the
post-WWII paradigm, best represented by Benjamin Spock and labeled
permissiveness, the belief was that children's errotic impulses could
be sublimated and redirected into exploration and learning, if parents
adopted a permissive and responsive mode of parenting. This is a very
schematic summary of a dramatic change. What it meant was that parents
were raising children by very different principles than they had been
raised and there is plenty of sociological evidence suggesting anxiety
about this transition. Many, such as Jules Henry, were harshly critical
of this new style of parenting, suggesting that it would be likely to
generate brats and monsterous children, while proponents constructed
images of children as naturally innocent and benign. Both images run
through the popular culture of the 1950s and may sometimes be merged
within a single figure, as occurs in the figure of Dennis the Menace.
The permissive paradigm came under increased attack in the 1960s and
early 1970s from conservatives who blamed Spock for creating the
anti-vietnam war protestors and the counterculture and calling for
stronger discipline to restrain children's base desires. The permissive
approach was also labor-intensive and assumed a full-time mother in the
home. As economic reality shifted, its paradigm gave way to a variety
of other alternatives and debates about appropriate childrearing were
beginning to be intense by the 1970s, when the later horror films come out.
I have mostly been looking at the 1950s films about children or for children
but there might be some link between the 70s films you cite and the
breakdown of permissiveness as the dominant model for talking about
children and their sexuality.
I offer this simply as a suggestion for further research and can
provide cites for some basic sources on these shifts if you want to
write me off-line.