Parents have described the agonizing moments they spent waiting at Tasmania's Hillcrest Primary School gate, unsure whether their child was involved in a fatal jumping castle incident.

Four children — two boys and two girls — were killed, and five others remained critically ill when a jumping castle on which they were playing flew several meters into the air.

Stressed parents were reportedly forced to wait at the school gates in the hours following the incident without knowing whether their children had been hurt.

Some panicked on social media, fearful of being kept in the dark.

One mother reported that she arrived at the school about 11.30 a.m. but was denied entry along with other parents.

"I'm here now and they won't let us in," she tweeted. "It was [child's name] grade and no one knows who was hurt yet."

Another stated that they had not heard from a friend whose child attended the school.

"What grade do you feel it is? I have a buddy who lives there with his children and he has not heard anything yet "They said.

She recalled a mother whose child attends a different school who drove to Hillcrest to show her support for a friend but encountered a "horrific" situation.

"I went there because a friend was upset trying to figure out if their kids were okay before she got there and it was just horrific," she wrote in a remark.

A man stated that he witnessed personnel feverishly attempting to save one of the children's lives.

"I was present when the emergency response staff was desperately attempting to resuscitate the first child... I'm certain they couldn't have done more. Regrettable, "He posted a status update on Facebook.

Adults responsible for another small child believed they could have received a different call if they hadn't dropped him off late for school that morning.

Following the incident, police encouraged the public to avoid the school, deploying two rescue helicopters and many ambulances to the location.

A tiny youngster who witnessed the atrocity claimed he narrowly missed boarding the ride.

"It was our turn next," he explained to The Mercury. "Grade five and six went first."

On social media, the school urged parents to pick their students "as a matter of urgency."

"Our school has been the scene of an accident. We're going to close the school for the remainder of the day "Its article was read.

According to Tasmanian police, "several children fell from a height of about 10 metres" after a "wind event" lifted a jumping castle into the air.

Scott Morrison, the Prime Minister, has called the situation "unthinkably heartbreaking."

"Young children are enjoying a pleasant day out with their families when it devolves into such a dreadful catastrophe. It hurts your heart at this time of year,"'He stated.

"And I just want to say, on behalf of all Australians, to the parents and families and friends, all who were there, to the other young children there and witnessing these events, I just pray you'll have great family around you and great friends and you can come through this horrific tragedy."