A child track is a regular track located inside a group track (double click on a regular track and check “group track” to convert it). You can move regular tracks to group tracks and those regular track will be child track.
Its use is audio routing, you could have two child track inside a group track, one of them with a BassDrum and another with a Snare, then add a distortion in the group track and both child tracks would be distorded.
Group tracks can also “just” be used to organize tracks if you have a lot of tracks. I uploaded an example video a couple of days ago which uses group tracks:
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