SHIJIAZHUANG, China—Hundreds of Chinese parents, some cradling infants, converged on the company at the heart of the tainted baby formula scandal Thursday, demanding refunds and asking what they can safely feed their children.

Thousands of others filled hospitals, many hovering over sons and daughters hooked to IVs after drinking milk powder tainted with melamine, a toxic industrial chemical that can cause kidney stones and lead to kidney failure.

The scandal highlights the changing family dynamics and economic growth in China. A generation ago, women had little choice besides breast-feeding. Now, supermarkets are filled with dozens of brands of baby formula, marketed to women who work outside the home after they give birth.

Yao Haoge, an 11-month-old baby with two large kidney stones, had been drinking Sanlu formula since she was born because both of her parents work. They had been puzzled by their little girl’s fevers and dark urine, but it never occurred to them that she had kidney stones brought on by her formula.

Now, like many of the babies at the Peace Hospital in Shijiazhuang, Haoge has an IV drip hooked into a vein in her head.

“We don’t make much money, but we wanted to buy good milk powder,” said her father, Yao Weiguan, a day-laborer from a small town an hour’s train ride from Shijiazhuang.

“We thought it was good and now it’s given us problems. And we spent quite a bit of money too.”

Baby milk powder laced with melamine, used in plastics and fertilizers, has been blamed in the deaths of four babies. More than 6,000 others have been sickened. Some 1,300 babies, mostly newborns, remain hospitalized, with 158 suffering from acute kidney failure.

On Friday, China’s quality watchdog reported that the tainted product crisis has extended to liquid milk.