How long should a child's scarf be?

It depends entirely on how tall this particular 5 year old is. Probably between ⅓ to ½ his hight. Call up his parents and ask for current hight, and then add a few inches to make room for a growth spurt. 58 inches is probably a good length.

I would use the height of the child plus 5 inches because I knit a hood in the center and so it makes the scarf look nicer and is warmer and there is a safety factor to consider here. The hood will keep the scarf from getting too tight around the neck. By the way I have five grandchildren that I have knitted and sewed and crochet for.