Police say four or five of the children complained of minor back pains as they were being evacuated from the bus Monday in Loves Park, about 80 miles northwest of Chicago.
The National Weather Service has confirmed that a tornado touchdown nearby.
Violent storms in Winnebago and Boone counties toppled power lines and trees, but authorities said there had been no other early reports of injuries.
ComEd utility spokesman Paul Callighan says some 42,200 customers lost power from northern Winnebago County east to Belvidere.