It was taken on Saturday, between about 2pm and 3.30pm.
There is a 6ft high wall around the garden and his mother, Stephanie Heaton, believes the thief must have climbed over to get in as the gate is bolted from the inside.
Only Jack's bike was stolen from the garden, despite his younger brother Connor's bike being alongside it.
The BMX was the West Minster Primary School pupil's Christmas present two years ago and Ms Heaton said he had had his heart set on it for a long time and he is devastated.
She is hopeful someone may have noticed the thief climbing over the wall and is appealing for people to come forward if they have any information.
Ms Heaton said she was in all day except for the 90 minutes when she thinks it must have been taken as she had gone into town.
The 31-year-old added: "He's pretty heartbroken that his beloved bike has gone.
"I'm wondering if someone saw him out on it and followed him back and stole it as they didn't take my younger son's bike."
The bike, which is worth about £100, has a frayed brake cable on the back and is rusty at the front and the handgrip is broken on the very end.
Sgt Stefan Martin, of the Island Neighbourhood Team, said: "These thefts can be very opportunistic so we always promote the use of a good quality cycle lock to deter thieves.
"We also recommend that cyclists mark their bikes with a UV pen.
"This means that if we recover a stolen bicycle it will be easier for us to return it to its rightful owner.
"The bike that was stolen was marked so that will help with identifying the owner should we recover it."
- Anyone with information about the theft is asked to contact Kent Police on 01622 690690 begin_of_the_skype_highlighting 01622 690690
Gardien Tip: See our article on Bike Security at http://www.garden-security.co.uk/bike-security-print.htm