They flung toys around and smashed them against one another.
They also created a ruckus, yelling and screaming.
The boys were not in the privacy of their home nor did the toys belong to them.
This ugly scene took place at the toy section of the Metro store at Compass Point.
The incident was captured on video and shared in a Lego enthusiasts' Facebook group on Saturday.
It led to a storm of criticism over the boys' behaviour.
In the clip, no one was seen telling the boys to behave.
It is not clear how long the boys went on destroying the toys, but the video is three minutes long and ends only when a man, who appears to be the boys' father, returns for them.
He is seen carrying four boxes of Lego toys.
Netizens were critical of the boys' behaviour. Some called them spoilt brats. Others held the parents responsible for the children's destructive actions.
Netizen Penelope Wee wrote: "Have to blame it on the parents. This is really poor upbringing. These kids need to be told off on the spot... Else they will never learn..."
Parents whom The New Paper spoke to said that the children should not have been left to their own devices.
A mother, who gave her name only as Mrs Lim, said: "If those were my kids, I'd slap them."
The 34-year-old executive, who has a two-year-old son and is expecting her second child, added that it is ultimately the parents' responsibility to discipline their children.
"The children are very ill-disciplined and inconsiderate. It doesn't reflect well on their upbringing," she said.
Housewife Melanie Goh, 40, said that children tend to be rowdier when they are left unsupervised.
The mother of three children, aged between 11/2 and seven, said: "I've also seen some who fight and snatch toys from each other. It's safer for parents not to leave their kids alone."
Other parents were more sympathetic.
Accountant Lynn Chia, 33, said that children might be rough with toys as they do not know better.
She said: "Sometimes, children are just clumsy and spoil toys unintentionally, especially when they get too excited."
Ronald Chan, 42, who works in human resources, agreed that parents are not entirely to blame.
The father of two sons, aged three and four, said: "Sometimes, parents have to rush off for some urgent errands, and they leave their children alone without knowing that they will cause so much trouble."
William Wan, general secretary of the Singapore Kindness Movement, said that while the video clearly shows the two boys misbehaving, it is unclear if they have mental health issues or conditions like autism.
Even then, parents are not excused from leaving their children to their own devices, he said.
Psychiatrist Alvin Liew pointed out that getting excited over toys can be part of a child's normal reaction.
So it is important for parents or caregivers to teach the child that the toys in a store are meant to be sold in their original packaging and should not be handled roughly.
"If children are allowed to behave rowdily when rowdy behaviour is not appropriate in certain social situations, they may get the wrong idea that they can behave in any manner as long as they wish," said Dr Liew, who has previously helmed the child and adolescent psychiatry department at the Institute of Mental Health.
Both Dr Wan and Dr Liew emphasised the role of parents when it comes to encouraging appropriate behaviour.
This is so children will learn how to "behave as they continuously interact with their external environment", Dr Liew said.
Dr Wan added: "It is a trite saying that graciousness must begin from young at home.
"Unless parents take the trouble to model and teach considerate, courteous behaviour, it is hardly unexpected that kids do not know better."
Rather than focusing on the boys' bad behaviour, he advised viewers to reflect on how to better ensure that their own children do not misbehave.
Metro declined to comment on the incident.
This article by The New Paper was published in MyPaper, a free, bilingual newspaper published by Singapore Press Holdings.