Grant was thrilled that his little girl's first word was "dad", especially with Father's Day just days away.
He poked fun at wife Chezzi Denyer in the caption, writing: "P.S sucked in @chezzidenyer we have a clear WINNER cause it's VERY clear that it's 'Dad' she's saying.
"Which of course we all know means, she officially loves me more. I'm sorry you had to find out this way 😎"
Of course, Sunday may have had a little bit of help from her dad, Grant sharing his secret technique to teach his daughter the word.
"It's certainly not the hours of me saying it to her when you're not looking that's responsible. Honestly," he joked.
"It's obviously straight from her heart and the heart wants what the heart wants. Who can argue with that cute face?"
He has a point – Sunday is downright adorable!
She's the youngest of Grant and Chezzi's three daughters, and as far as we know Sailor, 10, and Scout, five, adore their little sister.
Of course, with Father's Day coming up on Sunday, it will be Grant who will be soaking up the love and attention over the weekend.
He'll probably still be basking in the joy of his baby girl's first word when the big day rolls around, if the last lines of his Instagram post are anything to go by.
"Once a Daddy's girl, always a Daddy's girl. Sorry Chez, you can't beat true love or biology," he said.
"Or at least you can't beat my intensive secret training sessions where we practiced it over and over."
This story originally appeared on our sister site, Now To Love.