When he was just 20 years old, Tim was told that not only did he have a brain tumour, but the medication he'd need to take for the rest of his life would mean he had only a one per cent chance of ever fathering children.
"Despite life and death, despite taking medication every day for the rest of my life, that was the thing that broke me," Tim, 26, tells TV WEEK.
"The doctors told me not to bother with using contraceptives as there was no point."
Luckily for LEGO Masters contestant Tim, he had his high-school sweetheart Dannii, 25, by his side and could rely on her love and support.
"I remember having a conversation with her and asking, 'You've always wanted kids – are we still doing this?'" Tim says.
"Lucky for me, she was optimistic, and had always loved the idea of being an adoptive mother."
And Dannii never wanted to imagine a life without her best friend.
"If I'm being honest with myself, I never really believed Tim wouldn't survive," she says. "I was a bit naïve."
But as fate would have it, just as Tim and Dannii set out to plan their life without kids, the day they put down a deposit for a wedding venue was also the day Dannii found out she was pregnant.
"By some sort of miracle, it worked out," Tim says with a smile.
"I don't know how – and even the endocrinologists I see about the medication I take are stumped. And it worked out twice over!"
Tim and Dannii are proud parents to two-year-old Karliah Grace and 15-month-old Zaidia Dawn.
They hope their daughters will be proud of them for following their dreams by taking part in LEGO Masters.
"The girls will either think we're the coolest parents for doing what we did, or we're the lamest," Tim says with a grin.
"Either response will give me lame-dad pride."