Try buying slim jeans (not slim fit or slim cut) with the elastic strips in the waistband. The slims are usually a couple of sizes smaller in the waist than the length and you can make the waist smaller yet with the elastic straps. Both Walmart and Gap Kids sell slims.
Take them shopping and have them try different sizes on. I have a 25 year old grandson that has a similar problem. He is 6'8", needs a 38-40" inseam and a 34" waist. They aren't easy to find and you don't find them at discount stores either.
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