The children were playing at the stepping stones in the river, also known as the River Tame Crossing.
Located just off Uppermill Park, it’s a popular spot along hiking routes in the area.
While they were there, the eagle-eyed youngsters spotted an American signal crayfish crawling from between the stones in the river.
The crustacean can be identified by the distinctive white/turquoise patch at the hinge of its claws, which were sticking out from the water.
It is the third time this year that a sighting of the species has been reported, with one found in the Huddersfield Canal in April, and one spotted in the River Beal in September.
Now, the Mersey River Trust has confirmed it is aware of the creature’s presence, and is actively taking action to tackle it.
The American signal crayfish is considered dangerous to its environment, causing harm to local food chains, species and river banks.
The creatures actively hunt their cousins and the native species in the UK, the white clawed crayfish, which is much smaller and more docile.
A spokesperson for the Mersey River Trust said: “We are aware of the American signal crayfish in the River Tame, which is an invasive, non-native species from North America that was introduced to the UK in the 1970s.
“It outcompetes and spreads crayfish plague, which devastates the native, white-clawed crayfish, and its burrowing activities can cause significant erosion to riverbanks.
“Distinguishing features include its larger size and red undersides claws with a distinctive turquoise/white blotch at the hinge.
“Signal Crayfish can breed and multiply quickly and cause extensive damage to banks through burrowing.
“They reduce overall water quality and eat almost anything, from aquatic plants to small invertebrates, fish, frogs and even each other.
“We are currently working in partnership with the Environment Agency and United Utilities to help protect a small population of native white clawed crayfish within the Saddleworth catchment.”