childjobs Property error

When I try to execute exercise from MOL video :
Chapter 12 - Working with Child Jobs.mp4

Receive-Job -id 15 | select -ExpandProperty childjobs

I get error:

select : Property “childjobs” cannot be found.

Receive-Job allows you to display output of current running jobs.
The output is type System.String. Select the child jobs from ‘Get-Job.’

Get-Job -Id 15 | Select-Object -ExpandProperty childjobs

ah I see, thanks random