"At least 10 are injured. Ten, 15, 20 are injured, I don't know," Cleymans told the Reuters news agency.
The suspected attacker has been arrested, the interior ministry spokeswoman said.
Speaking to Al Jazeera, Teri Schultz, a freelance journalist in Dendermonde, said people in the small town were shocked by the incident.
"The man arrived on a bike and randomly attacked both kids and caretakers. The children at the centre are very small, three years and under," she said.