I clean and organize folks IRL and I have to say, the clothes issue is always the biggest problem. I have a client right now that I say her master closet is the "last frontier" LOL...anyway, I do have a rule of thumb for kids.
The following will give you enough to have if you don't have time for laundry one week.
Two weeks of OUTFITS (not individual pieces) from the skin out per season. This includes underwear, socks, and undershirts.(if you need them) If you just do the jeans and shirt routine, you can cross seasons by wearing short sleeves with jeans in fall and spring.
One week of pajamas or nightgowns.
One pair of athletic shoes, one pair of nicer shoes that can be worn to school.
One winter coat, one sweater, one hoodie and a couple of sweatshirts/ sweaters for layering.
A couple of winter hats and gloves and one pair of boots if needed in your area.
Now if you have church - add five OUTFITS and one pair of shoes per season. Otherwise, one nice outfit with shoes per season is fine.
Of course if you have sports - you will have to up the amount of underwear and socks.
ETA: Forgot bathing suits (2-3) and any snow pants or whatever is particular to your area.
The biggest rule is: Always buy in OUTFITS, not pieces and you won't end up with too many shirts and only one pair of jeans.
Hope I have helped someone. This has always worked well for me.