Hi Marzi. It's I didn't know you had a kid. When the verb in the main clause of the tense is in the past, so is the verb in the noun clause. Although the meaning of the sentence is 'present' even though the grammar is past. Siobhan :)

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Charles is correct. Any of the following noun clauses are grammatically correct. I didn't know that you had had a child. that you had just had a child. that you had a child. that you were going to have a child. that you would have a child. that you have a child. that you are going to have a child. that you will have a child. As a further comment, both examples "... had a child" and "... have a child" are correct because there is the option to not backshift the verb tense if the situation is still true. More information: https://www.ef.com/ca/english-resources/english-grammar/tense-changes-when-using-reported-speech/ [excerpt] You do not need to change the tense ... if the original statement was about something that is still true, e.g. We explained that it is very difficult to find our house.

Charles is correct. Both are OK. You picked an answer too soon.

Both are correct. After "I didn't know (that) . . ." you are able to express both past and simple present tenses. Technically speaking, these are called noun clauses and this case (that) is omitted and the remaining part of the sentence actually functions as an object. Question: I didn't know WHAT? Answer: you had/have a child.