I will leave it to you to write the poem, but let's talk about some ideas you might consider. Plath wrote the poem about the birth of Freda—you. A stanza you might want to respond to if you are Freda is the following:
I’m no more your mother
Than the cloud that distills a mirror to reflect its own slow
Effacement at the wind’s hand.
How does Freda react to the idea that her mother feels so alienated from her, given that a few years later her mother commits suicide? Does Freda feel like Sylvia Plath's daughter, given that she hardly knew her mother? Does she wish her mother had been more connected to her? Freda might respond to the verse along the lines of the following: "You were my mother. You were more than a cloud, and I needed your solidity, your presence as a physical being in my life. You say you were a cloud, but I needed a rock."
Plath likens Freda to a cat, to the sea, and to balloons. It would make sense to use those same images in a response. For example a cat jumps and pounces and claws--does Freda have any desire to be a cat in that way—or does she wish she could be a cat in the sense of curling up into her mother's lap? Or both? Does she wish to crash angrily on her mother's head like a fierce wave for leaving her or does she wish she could envelop her mother like the warm sea—or both? Perhaps she wants to pop like a balloon to get her mother's attention and bring them both back to earth.
In sum, I would think Freda would experience both anger at her mother for leaving her and longing to be with her, and both could be expressed in the response poem, mirroring back the same images Plath uses in different ways.