Here's my take- which some people might find extreme!
The problem of a disappearing childhood is more complex than societal influences. Someone mentioned how every older generation will say this about the current- times have changed etc etc. This is a good point. "childhood" is actually something very modern. Up until the 1900s people that today we would label children were working in fields and factories and getting married and raising families. It's our luxury and success that allows us to extend our "childhood". It's why some of us can still 'play' -- take time off of work etc to ridiculously enjoy Disney while other people in other places are starving or facing all sorts of hardships. So 'childhood' in one sense is a cultural issue.
Another way to define childhood is strickly going by peer group--- ignoring what our 'standards' are for a child to be having a 'childhood'. IE many people have a problem with children dressing too 'adult'- wearing shirts that show their belly buttons or low rise pants-- but this isn't a new phenomenon; children have typically always liked to play dress up and wear current fashion-- the question is are the stars of the day charlie chaplin or brittany spears? Either way this *is* kids stuff.
As a peer group you can expect this behaviour.
Other childhood based markets have recently found themselves floundering. FAO, toys r us, KB etc have all had financial troubles and noted 'problems' within the industry. It's ofetn said children are growing up 'faster' and no longer want toys-- or the traditional kinds of toys they are selling.
But this isnt all there is to it. There's also a crisis in family going on. What happens when children have more adult company than they have child company? There is less time for fantasy and childish playing, adults do 'force' a child to adapt to the amount of 'play' an adult can handle, and then try to hook the child onto adult interests that the adult will enjoy.
You might not be following =) But humans being a more tribal people, typically had more extended family and specifically more children than the average of 2.5 (and declining) that families have today. In large families the children do get to play more-- there are automatic friends and endless hours of fort play, more fantasy, more people to act out, to interact with, to share ideas with etc. So, in a sense contraception has 'ruined' the fraternity of children. Adults rarely let themselves be outnumbered by the children in their household so they are under a new kind of constant watch, pressure and rigidity that doesnt breed silly play =)