Mrs Withington says as they waited for firefighters to arrive, Sophia was thinking of ways to rescue her brother and decided to get the ladder from their bunk beds.
Timaru senior station officer Trevor Karten says Sophia greeted them at the gate and showed them where Elijah was.
"The bathroom floor was a bit rotten and the little kid fell through it. I'm not sure if the hole was there or if it gave way."
Mr Karten says Elijah appeared to have fallen about 150cm.
"The other kids in the family had grabbed the ladder off the bunks, stuck that down but there wasn't a heck of a lot of room so no one could get down there because it was only a child-size hole. When we got there the child was screaming a bit and one of my brave firefighters lay down on the bathroom floor and was able to reach down and grab his upstretched arm.
"Elijah had his arms up above his head and he pulled him out. We gave him back to [Mrs Withington], who was surprisingly calm."
Mr Karten says firefighters checked Elijah for injuries before St John Ambulance arrived and put a sticking plaster on a small scratch on his hand.
"He was a remarkably unhurt because a metre and a half is quite a drop.
"There was the risk of banging his head or falling on his head but he must have gone straight down."
But the firefighters' efforts didn't end there: they also fixed the hole.
"I went to the house next door and grabbed a pallet and patched the hole up."
The brigade also put up two long-life fire alarms in the children's bedroom.
Mrs Withington says she's grateful for everything.
She says her two other sons, Jonathan, 7, and Matthew, 4, were excited by all the action and wanted to be firefighters when they grew up.
And renovating the home was now "top of the list".