The "sugar smart app", from Public Health England, works by scanning barcodes and revealing total sugar in cubes or grams.
Officials hope it will help combat tooth decay, obesity and type two diabetes and encourage families to choose healthier alternatives.
PHE says young children are eating three times more than the sugar limit.
Its new Change4Life advertising campaign, which includes the sugar app, suggests that on average children aged four to ten years old are consuming 22kg of added sugar a year.
That's about 5,500 sugar cubes - more than the weight of an average five-year-old child.
The app has been developed to raise awareness of how much sugar is contained in everyday food and drink.
It works on more than 75,000 products, offering a quick guide to help parents to assess potential purchases that may harm their children's health.