I usually do 25-30 for an adult. I am making a scarf for a nine yr old. I started with 20but it seems really small. I am using a garter stitch and knitting every row so I don’t have a pattern.
Generally about the same, just don’t make it as long.
I knitted a scarf for my grandson in Va as his Christmas gift. I used 32 stitches in a pattern stitch that made it loo like big wide ribbing.
It was a 4 row pattern of 1 Knit Row 2 Knit Row 3 K2 P2 Row 4 K2 P2 .
It made a very cushy feeling scarf. I knitted it to 48 inches and bound off in Knit 2 rows just like the cast on. I will let you know how he likes it when I hear from them in a few days.
I think it depends on the weight (gauge) of the yarn you’re using. Twenty stitches in DK weight might not make a nice, fat kid’s scarf, but 20 stitches in a bulky (Class 5) weight would be really nice!