The adult set the budget, but as the kid I had control over how that budget was spent, unless it was a toy my parents found objectionable.
For other categories, you could argue that the toy companies are selling a service: Make the kids stop complaining for a few hours. Naturally, an easy way to do this is to be the reason that the kids are complaining in the first place.
Also, good proportions of toys are not exciting to kids at all. Especially during Christmas and birthdays. They are swimming in sea of toys they will never actually play with, because it is simply not possible to play with that many toys.
Parents and grandparents are the ones who spend the money, sure. But the advertising is directed towards the kids.