Child Busses

Hi, I don’t quite get Child Buses. I am setting up for a 5.1 mix. I want to be able to monitor on headphones in Stereo, so I thought adding a Child Bus to my 5.1 bus would let me do that. But it seems to monitor on the stereo bus, I have to select Stereo Out in the audio tracks Out put, which then stops it going to the 5.1 channels.

I’m confused, what use is the Child Bus?

Thanks
Stewart Peters