They parked the car in the Tournament Parking carpark on Queen St at about 4pm and walked down the road to buy food, an icecream and a CD for Tayla.
But when they got back about half an hour later, they found the windows of the car smashed and Tayla's things rifled through.
Missing was one of Tayla's "bright, bright candy pink" bags which contained her New York City costume of a tuxedo, tails, bow-tie and a glittery waistcoat with sequins.
"And it had a sparkly sequinned hat on top of it, so it was clearly not a iPad or MacBook kind of a bag."
Ms Johns said some of the items from the bag had been scattered on the floor of the car which showed the thieves had been through it and seen it was a little girl's costume but had taken it anyway.
They had not gone through the glovebox, the boot, or anything else - except for the dance bags.
"They'd actually opened the bags and gone through them before taking them, so they knew it was a little girl's stuff," she added.
Ms Johns and Tayla searched the carpark for three hours in vain, in case the "cowardly thieves" had ditched the bag in a bin.
Included among the stolen items was the Baby-G watch Tayla's father had given her for Christmas.
"She was bawling her eyes out because that was gone," she said.
"She was absolutely devastated and she hasn't slept the last two nights because she's terrified we're going to be broken into at home."
Ms Johns has been telling her daughter that "everything's replaceable" to try to calm her down.
"But she doesn't buy that, because it's not replaceable."
Tayla has also been crying herself to sleep in her mother's bed and needed to take Monday off school because she was so upset.
"She just can't quite fathom why someone would take her stuff.
"I guess she could understand if it was my stuff and it was expensive things, but she can't quite figure out why someone wouldn't leave her stuff alone - especially her little sparkly hat."
Ms Johns begged the thieves to dump the bags where someone could find them, such as at a local store or police station, so they could be returned to Tayla.