Baseo, in Southend, posted a film of the incident on its Facebook page, saying the child was used as a "visual shield" during the theft.
As reported in the Echo, a Samsung Galaxy A3 worth Â£151 was taken on 2 July.
Essex Police said it was investigating and appealed for details about the man.
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In the footage, the young girl can be seen perching on the edge of the shop's counter while the man stands beside her.
When a member of staff walks out of view, the man can be seen leaning over the counter to take something from a shelf.
He then appears to put it in his pocket before lifting the girl from the counter and putting her in a buggy.
The girl jumps out of the buggy to stand with a woman, while the man heads towards the exit.
Adam Keys, who runs the electrical shop's social media accounts, said the theft was "absolutely disgusting".
"The crime itself is enough, but to use a child to obstruct the view is something else completely.
"It's vile."
An Essex Police spokeswoman said: "Anybody who recognises the man, who is white, of a skinny build, in his mid to late 20s with dark hair, and was wearing a grey polo shirt, is asked to contact police on 101."
CCTV footage of a "vile" thief apparently using a child as a decoy while he steals a mobile phone from an electrical shop has been released.