The victims are Omar Ali, 15, and Jahid Hasan, 12, sons of Atiqur Rahman of Atapara in the upazila.
Witnesses said a group of boys was playing a football match on the char.
At one stage, their football fell into the river and Omar and Jahid jumped into the river to bring it back.
However, they went missing, they said.
On information, firefighters rushed in and launched a rescue operation, said Sariakandi Fire Service Station Officer Al Amin.