My boys always wear the trousers in sizes 6/7 and 8/9. I ordered their standard size small and medium which they always wear and figured the extra length wouldn't be a problem as my boys are leggy. Spring. people don't lie. They are LONG. So my child in size 8 Small fits perfectly and my child in size 6/7 XS fits well. They're still a long way off, but that's good for us. If your child has short legs I might skip them altogether, but otherwise just scale them down.
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