"If" with implicit "then" question: Should a comma be used?

In that case, the comma is optional. According to the Wikipedia page on comma:

In English, a comma is generally used to separate a dependent clause from the independent clause if the dependent clause comes first: After I brushed the cat, I lint-rollered my clothes. (Compare I lint-rollered my clothes after I brushed the cat.)

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While many style guides call for commas, many authors omit them, particularly with short sentences.

The two paragraphs that I extracted from the article apply to your sentence. It’s a short sentence with a dependent clause (if the item is an apple) and an independent clause (it should be given to children), where the dependent clause comes first.