As a mum with a kid who's in 7's for length and 2-3 for knickers I totally understand the frustration!
Sadly the only thing I've found is adjustable waists. Target tights/leggings are fairly slim fitting and seem to be ok. Often she wears tights and dresses, and also I have put simple darts in the tops of her pants.
I fold the waist over. DS still needs size 1 pants for the waist, but they're too short now...
Some pants I bought 2nd hand I sewed extra elastic in to make them tighter.
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