My husband Came in 2016 on B2 and change the status to F1 sponsered by his sister. Me n my 6yr son came in jan 2017 and changed my status to F2. Later i got admission in PhD and changed my status to F1 which will be effective from August.
I am not sure we would be able to answer this. You can apply at any time you or your husband is on the F1 and the child is under 21. Whether the consulate of oblige to grant the visa is a different quesiton all together. One the consulate should be asked to answer.
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