Tweaking one thing then another is the wrong approach, as the adjustments are interdependent.

First check that the derailleur hanger is not obviously misaligned, such as by dropping the bike on its drive side; kids do this a lot. If the hanger is not straight you will have problems making adjustments correctly. Fix this first.

Follow this procedure from the beginning, without skipping any steps. Do not proceed to the next step before the current one is correct, as they depend upon previous steps being right.
https://www.parktool.com/blog/repair...eur-adjustment
and the front: https://www.parktool.com/blog/repair...eur-adjustment

Make your adjustments gradually, in 1/4 turn or less increments, so that you do not miss the sweet spot. Even 1/8 turn can make the difference between so-so and excellent shifting.
Unless something has gotten replaced or broken only small adjustments should be needed; these bikes presumably were ridden previously so should not be that far out of adjustment.