The jp_kids is a child language of jp. That means that all the strings that it contains only the modified strings from jp.
This makes maintenance in the long run a lot easier: think of 1000 - 1500 new modified strings per Moodle version, twice a year. Most of those strings are not user interface strings and would not need any change for the jp_kids language pack.
I suggest to use the search capabilities on https://lang.moodle.org to identify what really needs modifying and only change that.
A lot will come with using the language pack too. If you see something you don't like, you change it in the kids language pack. Your site by default updates language packs every day, so the next day your changes are life for everyone...
This page: https://docs.moodle.org/dev/Translation_priority gives you some clues of what is interesting to work on and what not.
A typical kids language pack contains a few hundred modifications, with Hebrew kids as a top of 1200 modifications. That's far from the 20000+ strings a language pack contains.