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I'm a curriculum developer and write training materials for an agency that works on behalf of children. I often use "child(ren)" when the context could refer to one child or more than one child.

Which is correct: "The child(ren) is.." or "The child(ren) are..."? I can't find any grammar rule anywhere that relates to that issue.
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keneldeerWhich is correct: "The child(ren) is.." or "The child(ren) are..."? I can't find any grammar rule anywhere that relates to that issue.
I haven't seen this before but it strikes me as a perfectly good extension of the much more common "parentheses s" as in father(s), mother(s), etc.
Comments  
'children' is the plural of 'child' so:

The children are ....
The/A child is ....
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 RayH's reply was promoted to an answer.
MichalS, that doesn't really answer the question. There are times when I could write "The child/children is/are..." but that is too awkward. Instead, I could write "The child(ren) is/are..." but that too seems awkward. Would it be proper to say "The child(ren) is..." or "The child(ren) are..." to avoid using the slash?
The child(ren) are. child(ren) is read as child or children - a singular subject connected to a plural subject. This means you should use a plural verb.
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Child is singular. Children is plural. Therefore, use singular verb for first (is) and plural verb (are) for second.

The child(ren) is(are)...

are.

You are technically saying "the child or children..." therefore are would be most appropriate.

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