You could look into mental health therapy, they cover a ton including things like parents breaking up, grief, emotions etc, I think some schools now have a councillor there but I don't know about preschool, some of our preschool/nurseries are connected to the primary school but if yours isn't or they aren't at preschool/nursery then you could maybe look online at what is offered in your community an see if there's anywhere that could help especially because they are so young, I know at my business it's from age 7 up we will only see
But advice wise, things I've picked up from working in this type of environment is make sure she knows this isn't because of her, talk her through it, allow her to voice everything, an when the time comes that your partner moves out, completely redo her room, allow her to pick what she wants an go for different colours, bedding, curtains etc, memories are also connected to smells so completely change her bath stuff, shampoo, make it a fresh start for her, it might help, I know it sounds weird the whole bath thing but if your partner has been doing bath time an they are no longer there the stuff they used can subconscious bring back memories through smell so she might get upset but won't understand why she's upset