What is the criterion for the trifling amount for which it is permissible for a child to sell something?

"Scholars have differed regarding the validity of buying and selling by a discerning minor. The Shafi'is hold that it is absolutely invalid, while others, such as the Hanafis and Hanbalis, permit it with the guardian's permission. They also permit the minor's transaction in minor matters without permission.

As for the non-discerning minor, his transaction is not valid except in very trivial matters. What is meant by 'trivial matters' is that which people customarily consider trivial and which is commonly purchased by children."