Sharing a post on Facebook, Leeson simply wrote 'My son drowned 3 days ago. His limp, gray, lifeless body was pulled from the pool and it was every mother's worst nightmare.
The mum-of-five went on to explain that a million thoughts went through her head as she stumbled around screaming.
'I saw my dead baby on the ground. I thought about his twin brother and how could his life go on without his twin. I saw my 10-year-old son, hysterically sobbing, in his bright blue swim trunks, his beautiful tan skin glistening with pool water still. His life ruined because he just watched his brother die, drowned in the same pool where he was playing.
'I saw my sassy little 3-year-old daughter, in her pink unicorn and rainbow suit, just watching me, confused. And how was I going to tell my husband that I let out perfect 5-year-old son drown??'
As Kleeson struggled to come to terms with what she had seen, one of the other party-goers had thankfully begun CPR, ultimately saving the boys life.
However, the distraught mother thought that even if he could be saved, it was too late.
'I was sure machines were going to be doing his living for him,' she wrote.
Incredibly, little Adam has made a full recovery and has even been swimming again with his mum.
In sharing her post, Leeson wrote that the reason she has shared her story so publicly is to raise awareness of child drownings.
'This happened in a pool full of people. A pool full of ADULTS,' she wrote.
'I've read so many stories about kids slipping away from their parents and getting into a pool, to be found drowned shortly later. I've never considered the possibility that my child could drown right in front of people who were watching him bob up and down from the bottom of the pool to just below the surface, but didn't think he was struggling because he looked like he was PLAYING'.
Leeson continued by saying she could understand why the adults around her boy didn't recognise that he was drowning, admitting that she too thought he was just playing.
'This was 100% preventable. The fault was MINE,' she added, explaining that there are three key points she has taken from the incident.
'I'm sharing this because I want to prevent this from happening to anyone else. Before going to any pool, first make sure your kids know not to get in until the adult who is responsible for them is ready to watch them.'
'Second, KNOW THE SIGNS OF STRUGGLE!'
'Third, know CPR.'