A happy childhood is a form of what Owen Barfield called Original Participation; that is to say, it is a mostly unconscious and spontaneous immersion-in the society of the family and more generally. The child feels a belonging that it not separated from 'the world'.
When a childhood is happy, adolescence is likely to be a threat to that happiness; because of its psychological and spiritual separation from parental values, and the need to choose whether to accept or reject parental values; the loss of spontaneous engagement with the world; and an increasing self-consciousness. With adolescence there will be (whether threatening, or actual) some significant degree of isolation and loneliness.
The happy childhood may lead to the attempt to reject adolescence in at least some of its aspects; a holding-onto childhood happiness - and this can work - to some degree, and for some time.
However, sooner or later, adolescence will drive-away the spontaneity of childhood modes of being, and it will be discovered that they cannot be simulated convincingly.
Therefore, adolescence will eventually separate us from the Original Participation of childhood, and precipitate us into the alienated state of the Consciousness Soul, as it is termed - which is the adolescent state-of-being of an individual, and also of our society.
The pathological spiritual state of this modern (especially Western world) will then tend permanently to trap the maturing individual in this alienated state of a permanent adolescence; and modern people will be socially-encouraged to seek adolescent gratifications.
(Hence the nature of modernity; and its images and goals.)
Indeed, in a world of materialism and spirit-rejection; a world where the divine is excluded from all public discourse and plays very little primary role in religious discourse; it requires a personal, individual, inwardly-motivated "quest" even to seek beyond the short-termist gratification - but purposelessness and meaninglessness - of modern pseudo-adulthood.
The deficiencies of this state become more and more apparent with advancing age and the onset of old-age (when we recognize that feeling, looking and behaving younger is the only publicly-approved ideal); although the harshness of alienation may well be ameliorated considerably, albeit vicariously and temporarily, by creating a happy family life, and participating in the situation of a happy childhood for one's own children.
But the temptation of an idealistic yearning for a return to a happier early life (or even an imagined happier earlier life - i.e. childhood as we know innately it should have been, and sometimes is), is a yearning temptation that will seldom diminish - unless some kind of a spiritual revolution and the goal of a state beyond perpetual adolescence is accomplished.
Hence, a happy childhood presents a problem in the world as it now is - a problem that does not go-away unless it is overcome; unless that early happiness can be understood as a foretaste of some higher state of being that is yet to come, and which is indeed attainable.
Some people tend towards a hoped-for return to that childhood state, or indeed a complete and permanent version of the partial and temporary childhood state, in the world after death...
Such are the desires for a paradise of unselfconscious, merely-being after death - entailing the dissolving of our plaguing sense of self, and the cessation of that adult thinking which separates us from life, the world, other people.
At extreme; the desire is a total rejection of consciousness and its curses; yearning for an afterlife without body, a life as a pure spirit that is not separated from the divine or is continuous-with the rest of the world... A bliss state, a comfortable sleep.
As it were, a return back through childhood to the womb, and beyond into non-being - non-separation. This entailing a recognition of the futility of this earthly life - handing-back the entrance ticket of incarnation into mortality, and acknowledging that this world is wrong, bad, a torment - something best left as soon as possible.
However, the happy childhood can be taken otherwise. It can be learned-from, instead of being either rejected (in favour of permanent arrested development in adolescence), or else something to be recapitulated (as some version of a future eternal and completed childhood).
Childhood happiness can taken as evidence of the possibility of happiness as a separate and incarnated being; an experience from which we can learn in order to move beyond and to something higher, more satisfying across an eternal timescale.
By my understanding, it is the possibility of an eternal higher happiness that is precisely what is on-offer with resurrection into eternal Heavenly life.
But that is only fully and finally accessible via the portals of death. So how can that be helpful to our life here-and-now?
The first thing to say is that the possibility of future post-mortal higher-happiness does not in any way guarantee present happiness in this mortal life. Indeed, the specifics of this mortal life - its degrees of happiness and misery, are as variable as the number of people alive and who have died.
To look forward in faith to salvation does not "make us happy" here-and-now.
But - such a prospect before us does make the miseries of this mortal life potentially worthwhile; and it is up-to each of us to realize this potential by learning from our suffering - as well as by striving-for and valuing our present happiness.