Thank you for your question and sorry for your loss. In our traditions it is reported that a miscarried child will refuse to enter heaven without its parents. The imagery that the Arabic word for refuses uses, is the same refusal that children have to sometimes the simplest of issues, where they will not budge until the grown up has to bow to their will. As such the loss of children at the stage of miscarriage is a gain for both them and you, and you will be reunited with them at the gates of heaven.
There are no specific narrated dua to prevent miscarriages that I know of, so recite the duas that you identify with and those which express your feelings. Seek closeness to Allah through understanding these tests as He only tests a person for their own improvement.
May you always be successful and may He provide you with plentiful offspring.