This is not your situation, OP, but I really do recommend that you tell nursery about the split, or indeed any big change at home, whether or not you think it is affecting your child. You don't need to give details, but just give them a heads up so that they can keep an eye on her, just in case.
I remember a child at school, aged 7, whose personality seemed to change overnight. From being a quiet but willing pupil, one Monday he became disengaged, irritable and suddenly exploded, throwing his book in the air and bursting into tears.
He wouldn't say what was wrong, but said noone had said or done anything to him, he just hated school and wanted to go home.
I rang his mum who was shocked and surprised but confirmed that she knew of no problems and would talk to him that night. She sent a message in the next day to say he was fine and she thought he was just overtired.
Except he clearly wasn't fine. He didn't have any more blow-ups but over a couple of weeks he became more and more withdrawn, quietly stopped working and stood around alone at playtime. We tried everything, 1-1 chats, pairing him with different children, giving him choices of activity, insisting on him completing work, asking him to take work home to finish etc. All to no avail.
We kept in close touch with his mum who said he was fine at home and not saying anything about school. She said there was nothing happening at home and that he wasn't ill.
One weekend I ran into his mum in town, hand in hand with someone I knew was not her husband (who had been to social events at school, but was never involved in conversations about his son at school). She introduced this man as her new partner who had moved in as her husband had moved out, exactly when her son had changed. 'One out, one in!' she said. I must have shown more than I intended in my face because she added, 'Should I have told you?' It hasn't affected him at all'.
Once we (sensitively, obviously) talked with him about his dad, he opened up and talked about how he felt about the situation. We were able to provide support and he was able to tell us when he couldn't cope or how we could help him cope. We couldn't change the situation, but us knowing about it made such a difference to him and to how we dealt with his response.