A toddler is in a serious condition after being found unconscious in a small river in Adelaide's north on Easter Sunday morning.

The little boy, aged under two, was discovered by his parents in the Little Para River at Paralowie just after 9am after going missing from their family home.

He had gone out with an older sibling but only one of them returned.

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By the time the youngest was found in the water, he was unconscious.

Paramedics raced to the scene and performed CPR.

He was rushed to the Royal Women's and Children's Hospital where he remains a critical condition.

South Australia Police are checking security camera footage.

9News understands the two little boys were filmed as they walked down along the street to the river.