While it is not clear how far the child had driven, a senior police source told the Herald that cases like this were becoming more common.
"There's been an anecdotal increase in reports of this nature and while it seems to defy good sense, I guess it proves how alcohol destroys judgment," he said.
The man, who now has three drink-driving convictions, will be sentenced in July.
Retired long-serving road cop Stu Kearns, who headed the Waitemata Serious Crash Unit, described the case as "ludicrous".
"There's no way a 7-year-old boy's got the skills to drive a motor vehicle.
"At 7, crossing the road is hard enough for them, in terms of judging distance.
"There's just no way a 7-year-old would cope with driving."
It was unclear whether the car was a manual or automatic vehicle.
"Most 7-year-olds I know, depending on the type of vehicle it is, would barely reach the pedals - and then I don't know how they would see over the steering wheel."