The Change4Life campaign urges parents to look for low calorie snacks and limit their children's snacking to two times a day.
The move comes after PHE research found that around half of children's sugar intake, of around seven sugar cubes a day, comes from unhealthy snacks and sugary drinks, including sweets, chocolate bars and cakes.
On average children consume at least three unhealthy snacks and sugary drinks a day, with around a third consuming such food and drink at least four times a day.
Around a third of children leave primary school overweight or obese, according to PHE, which hopes to persuade the food industry to cut 20 per cent of sugar from children's food by 2020.
"The true extent of children's snacking habits is greater than the odd biscuit or chocolate bar," said Dr Alison Tedstone, PHE's chief nutritionist.
"Children are having unhealthy snacks throughout the day and parents have told us they're concerned.
"To make it easier for busy families, we've developed a simple rule of thumb to help them move towards healthier snacking - look for 100 calories snacks, two a day max."
PHE has created a free "food scanner" app, to help show parents how many calories, sugar, salt and saturated fat is in their children's food. In addition, money off vouchers for healthy snacks are available via the Change4Life campaign.
Last year the coalition Obesity Health Alliance (OHA), which includes more than 40 health charities, medical royal colleges and campaign groups, criticised the government for not doing enough to tackle child obesity a year after ministers launched a strategy to address the issue.
The OHA's one-year on report card was particularly concerned that more was not being done to halt the "relentless exposure" of children to junk food advertising.