The pre-Broadway production of Porgy and Bess, at A.R.T. in Boston, used cast member Natasha Williams' twin infants as Serena and Jake's baby. However, by the time the production moved to Broadway, they replaced the babies with a doll.
I don't have an actual answer to your question and I am not entirely sure what rules are in place to govern the situation. But there are obviously many factors that make using a doll preferable to a real baby. A baby is liable to start crying uncontrollably or to soil its diaper in the middle of a scene. As with most child actors, I imagine you would need to have at least two babies to alternate performances. And they're not just props, like baby dolls, so you would have to pay them.
Updated On: 2/26/16 at 09:15 AM