If I've understood correctly, the text-multiline field contains the list of tracking numbers. You should be able to use 'Find all matches to a Regex' to extract the tracking numbers and then insert a loop in which you search for each child record using the and update it with the record ID from the record in the trigger.
I've used this approach to do something similar. The challenge (for me at least) was to form the correct Regular Expression. In my case (?<=~~).+?(?=~~) was used to pull out each string between separators of '~~'. To pull out text on separate lines the Regular Expression might be something like ^.+$, which looks for any number of characters between the start of a line and the end of a line.
Hope that helps a little.
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Jeremy Anson
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