Crudely put, the idea is that lead in gasoline meant lead in auto fumes, which mean lead in the air. Lead in the air meant lead entering the body through the lungs, which meant mass low-grade lead poisoning / developmental problems.

Less crudely put: http://en.wikipedia.org/wiki/Tetraethyllead#Toxicity

The above links to Mother Jones do a pretty good job. To make a long story short, the lead in the fuel came down to earth in solid particulate form, and formed fairly high concentrations in the soil, where it was ingested by kids.

Noting Crito's comment, I don't know if inhalation or ingestion was the primary vector.

I was under the impression that inhalation played a large role; suggested by elevated crime rates in urban areas where there was an abundance of car fumes but next to zero farming.

I may be wrong about that; I'll have to read more about this topic.

What I remember reading is that kids ingest a finite amount of soil in the course of simply being outdoors. In fact lead forms some compounds that are actually tasty, and I think it was in the time of the ancient Romans that they boiled wine in lead pots to produce lead acetate, an artificial sweetener.

But inhalation would make sense too.