Reward charts are a mainstay of modern parenting. Stickers are commonly used in order to tame toddlers, stop tantrums and encourage kids to eat up their food, tidy their rooms and work hard at school.

Apple has a sticky problem on its hands, and until it’s resolved, it’s throwing a wet blanket over the experience of millions of people using iMessage and its new App Store. Stickers are like emojis ...