A search was launched after two children were reported missing at Stanwick Lakes.
Police were called at about 2pm on Sunday after the parents said the pair had got lost on the river.
Officers and the fire service arrived to assist with the search, and an ambulance was on stand-by.
Police spoke to fishermen nearby to see if they had seen them and a local resident also offered to go out and look for them in a boat.
However, the search was called off after a police officer spotted the youngsters in their kayak from a bridge.
The boy and girl were then safely reunited with their parents at about 2.30pm.
A spokesman for Northants Police said: “People should be wary of going into rivers, which can be fast flowing, and keep an eye on children if they are going into the water.”