Thank you for reaching out to us.
We appreciate the sensitive nature of this question.
It is not permissible for husband and wife to sleep in the same bed (or in two beds that aren’t separated) during niddah, even with a child between them.
In this case, we would recommend looking into buying, renting, or borrowing a cosleeper attachment that you could attach to the side of your single bed during niddah, providing more room and safety for your baby while allowing you to continue to cosleep.
We hope this helps. Please let us know if we can be of further assistance.