My kid's bike has some gears that aren't working properly (rear derailleur). They "slip". That is, he's pedaling along and suddenly the gear slips, as if it is going to change gears, but doesn't actually change gears. This happens repeatedly. It doesn't just slip once and then stay, no, it continues to do it over and over again while pedaling without making any adjustments on the gear selector.
Another interesting symptom is if I try to keep track of which gear it is supposed to be in, and which gear it is actually in, I get a table something like like this:
Gear it's supposed to be in - Gear it's actually in 1 - 1 2 - 1/2 3 - 2/3 4 - 3/4 5 - 4/5 6 - 5
... where the "/" indicates that the gear state could be one or the other, sometimes depending on whetheur it was shifted into from the higher gear, vs shifted into from the lower gear (shifted up or down) and/or the two gears it is slipping between.
How do I go about tuning them?