Police, paramedics and firefighters were scrambled to Tranent Church Hall on Church Street just before 7pm on Tuesday.
Two seven-year-old boys were taken to the Sick Kids - one is understood to have needed staples in a head wound, the other suffered a sore neck.
“A leader was also in shock - this could’ve been catastrophic,” said a witness.
About 100 cubs, scouts and beavers were packed into the hall when a piece of plaster measuring about four metres across plummeted 30ft from the ceiling onto a table below.
One parent, also in the hall at the time, said: “The ceiling plaster collapsed from the roof over a group of children, the Beavers, and it was chaos - walking wounded and one with head split open. This could have been far worse.
“The Beaver leader was distraught, three Beavers were being enrolled just as the ceiling came down over all the group
The Scottish Fire and Rescue Service said it was not a structural collapse but plaster fell from the ceiling.
A number of other children suffered minor injuries but no-one was seriously injured.
The collapse is now being investigated with water leaking from the roof and weakening the plaster understood to be a possible cause.
A police spokesman said: “Police in East Lothian were called to a building in the Church Street area of Tranent at around 7pm on Tuesday 9 October following a report that a portion of the building’s roof had collapsed.
“Emergency services attended and a number of people were treated at the scene by the Scottish Ambulance Service for minor injuries.
“Two 7-year-old boys were taken to the Royal Hospital for Sick Children in Edinburgh as a precaution.
“East Lothian Council protective services have been informed and are making relevant enquiries.”
Hall owners Church of Scotland have closed the building while investigations are carried out.
A Church of Scotland spokesman said: “We have met with the Health and Safety executive this morning and the building will be closed until further notice.
We are very thankful that the injuries sustained were not more serious and we hope the children are now recovering. Our prayers are with everyone who was hurt or affected by this incident.”