CHICAGO (CBS) — They’re seats that sell for between $35 and $50 each, and more than 4 million have been sold in the past nine years.

Now, a Chicago-based group is calling for a recall of the Bumbo Seat, saying the product puts kids in danger.

Michael Fisher tickles his 3-year-old son. While the boy seems perfectly healthy, Fisher fears his son may be developmentally delayed.

“His speech is a little behind – I mean, more than a little,” he tells CBS 2’s Mike Puccinelli.

The father thinks it’s plausible his son has some sort of internal brain damage. The child was just six months old when he flung himself out of a Bumbo Seat, which was on top of a table.

Fisher’s wife was there at the time, and he was on his way back into the room when he saw his child begin to fall. He dove, but was a fraction too late.

“His head bounced,” Fisher says.

The bounce on the ceramic tile floor fractured the boy’s skull.

Emergency room doctors feared they might lose the boy. That’s why he was flown to another hospital.

Fisher says it should never have happened. He blames the Bumbo Seat and has retained a lawyer.

“This is about them changing their product,” he says.

He would like to see safety belts attached to the foam rubber seats. But officials at a Chicago-based advocacy group go further.

“Kids in Danger wants to recall this product so we can remove this potential dangerous item from the shelves,” Sarah Chusid, the organization’s program director, says.

Kids in Danger says the seats were recalled in 2007 so that larger warning labels could be adhered advising caregivers not to use them on an elevated surface. But the organization says that hasn’t stopped the problem, and they say the seats are dangerous even when used properly.

“Even when it’s used on the floor as intended, children are being injured and there have been many skull fractures,” Chusid says.

Bumbo International is a South Africa-based company that donates its profits to charity. It says the Bumbo Seat is a safe product for infants when it is used as intended: on the floor and never on an elevated surface.

Bumbo says children should always be closely supervised when they are in the seat. They say no child has suffered permanent injury from a fall from a Bumbo Seat being used on the floor.