It was not immediately clear which side fired the shell in the North Sinai town of Sheikh Zuweid, scene of deadly fighting earlier this week.
A woman and a teenage girl were also seriously wounded, the sources said.
The government called in air strikes on Wednesday after a spectacular IS attack in Sheikh Zuweid left dozens dead.
The army said 17 soldiers and 100 militants had been killed. But medical and security officials said the death toll was at least 70 people -- mostly soldiers -- as well as dozens of jihadists.