How should I say: "she will have a child" or "she will has a child" and why?

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Hello Jakub,

'Will' is part of the family of modal verbs and they are always followed by a bare infinitive (1st form - no 'to'). So 'she will have a baby'. Other members of this family include 'should', 'would', 'must' ...

You can also use both phrases given by Valery, both natural.

Hope this helps

Bob

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She is expecting a baby / she is pregnant

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"She will have a child" is correct.

I'm not an English teacher, but I believe the reason why is the future tense is expressed as "will" + [infinitive]
Examples: "he will walk" instead of "he will walks"
"she will give" instead of "she will gives"

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Thank you very much! :)

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She will have a child. 
After WILL and WON'T, use the base form (bare infinitive) of the verb.

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