As far as treatment goals, I'd start by asking her how she would like to be able to react when she sees a cockroach. Does she see any need to react to a cockroach differently than she would respond to a grasshopper or butterfly?

Does she see cockroaches as being dangerous in some way? If so, it would be important to address those fears cognitively in order to lay a foundation for exposure-based treatment.

Yes, graduated exposure to decrease her intense response to cockroaches makes a lot of sense. There certainly is no need for her to develop a fondness for cockroaches, but it sounds like it would be quite useful if she could calmly deal with the cockroaches she encounters. My bet is that is that exposure would work fine if it is framed as a way to tone down the intensity of her reaction but that it would also work fine if framed in terms of mindfulness where the goal is to be mindful of her thoughts and emotions then respond skilfully anyway.

If there are other signs of OCD, it could be quite useful to address them. Likewise, if there are any signs that her primary fear is of her parents being gone and that this is a way to get them to come home, this would be important to address.

It may well be possible to treat her fear effectively without addressing her mother's fear of cockroaches. However, it would be quite helpful if the mother could model a calm but effective response to cockroaches (i.e. clamly squashing the cockroach) rather than modeling a fearful response. It might be quite useful to involve the mother in treatment as well.