Is there any reason to assume that LEGO is really evil? I for one am thankful for it's existence, because my kid is having a lot of fun with it.
LEGO is not alone in this regard, but they do create wants out of nothing.
So, you have a child, and this child is happy. Then the child sees a LEGO set, the child wants it, and now the child has an unsatisfied want. Child gets LEGO set, set satisfies synthetic want, child is happy again, and welfare returns to its original state.
Now, you might argue that there is a net increase in welfare from start to finish, since the child may have learned something new or acquired new experiences. Ok, sure, but after the 4th or 5th set, you are going to see some diminishing returns. And in this regard, LEGO's interests and your interests are opposed.
Have you played much with legos? If there is a point of diminishing returns, it is far after the 4th or 5th set. Every set linearly increases the number of things you can have built at one time (and yes, that matters), and generally introduces some unique pieces that increase the space of possible models. If it's true that newer models have dramatically less composable pieces, then my argument is weakened, but when I was a kid the only way my sister and I could have had too many legos was if they didn't fit in my room anymore.
As for creating wants ex nihilo, well, yeah, but only to the extent that it's true of any company that advertises at all. Entertainment and creation come right after food on a child's list of priorities, sometimes before; generally this is not a synthetic want. I'd rather know about good products like lego than take whatever dreck I stumble upon.
LEGO is not alone in this regard, but they do create wants out of nothing.
So, you have a child, and this child is happy. Then the child sees a LEGO set, the child wants it, and now the child has an unsatisfied want. Child gets LEGO set, set satisfies synthetic want, child is happy again, and welfare returns to its original state.
Now, you might argue that there is a net increase in welfare from start to finish, since the child may have learned something new or acquired new experiences. Ok, sure, but after the 4th or 5th set, you are going to see some diminishing returns. And in this regard, LEGO's interests and your interests are opposed.