Interesting point. Kids typically go through several bikes as they get older, moving between the wheel sizes - 12.5" with stabilisers, then the stabilisers come off and the next bike comes along - 20" wheels, then there is the 24" wheel size and then something with the adult wheel size but still a kids bike.
This is what the bicycle trade want, however, it does not always work out that way. Families will hand bicycles down from big kid to little kid, on to neighbours or sell them on as second hand. That cuts a lot of sales potential out. Also some parents will insist on getting 'value for money' and have the child ride a bike that is too large for them so they can grow into it.
Roads are full of these killing machines called cars, because of this only a minority of kids are allowed bikes. Cycling is more of an adult thing, or adult with small children, whereas it used to be more for older children, giving them freedom and independence. Young adults also want to go by car, so sales could be improved in that demographic.
I would actually argue that kids result in more sales of cars than bicycles. A family will need to go through a few cars, one for him and one for her, to do all those family things like shopping, school drop-offs and whatever else comes along. Come 17 another car will be needed because child will have passed his/her driving test and must therefore drive everywhere from that moment onwards.