The party, whose data is based on local authority test purchase results, also alleges that too many retailers "get away" with making underage sales of tobacco and alcohol.
Retailers refute the allegation that they deliberately sell to children, and point out that Trading Standards test purchases are targeted at problem stores.
Association of Convenience Stores chief executive James Lowman said: "Any test purchase failures are regrettable and every retailer needs to get the best processes and systems in place.
"We have to remember that local authorities tend to test premises where they believe there are problems, so the test purchase failure rates are not based on a representative sample."
Liberal Democrat shadow communities and local government secretary Julia Goldsworthy said she wanted to see a 'one strike and you're out policy' directed against retailers. "Too many shopkeepers are getting away scot-free with selling cigarettes and alcohol to underage children," she said. "We need to send a strong message to those who are ignoring these important age regulations."
Her comments have also been slammed by the Tobacco Retailers Alliance. "The headline of 22% of shops selling tobacco to children is not only misleading, it is inaccurate," said spokesman Ken Patel. "Shopkeepers need to be recognised for the excellent work they are doing in reducing underage sales. Government figures show that youth access to tobacco from shops is at an all-time low, and has decreased significantly in recent years."