In reply to Phil1919: meant to add last night: the title is a clue to the double meanings:
Obviously it mainly refers to the lost child, fixed in time. Lots of references in the TV version to them still loving the child (who they imagine later to be an older child or even an adult), which in turn applies to everyone- we all remember our 'inner child' or parents pine for when their kids were younger.
But '*The* child in time' could also refer to how we socially construct childhood differently through history - as in 'childhood through the ages'. This is why there is not just the sub-plot about Charles but the bits about the Government inquiry with various policy wonks droning on about how we ought to bring up children. It seems to be Charles' sense of shame of his hypocritical role in this that contributes to the conclusion.