Hello, I'm so sorry to hear about your situation.
My parents are divorced too (I was late teens when it happened, many, many years ago) and it has raised issues with our children from time to time,even though it's all they've ever known with those grandparents. With such young children I think that a very straightforward explanation is needed. Something along the line of how sometimes adults stop loving each other and decide that they will be happier if they live apart. It doesn't mean that they stop loving their children or stop being a mum and dad. You may need to repeat the explanation over many occasions.
If you're with their dad, then I imagine it will raise worries for your daughter about whether you and he are about to split up too. I responded to those worries by saying that Daddy and I don't want to split up and we love each other very much. I didn't feel I could say we'll never split up as I can't promise that but saying that he and I loved each other and wanted to stay together seemed both honest and, I hope, reassuring.
I can imagine that not having siblings may place a bigger burden on to you. I would suggest that you have a series of groundrules in your head which, should the need arise, you make apparent to your parents. Hopefully they will have the sense to know these for themselves but you never know and people can behave badly in times of stress. However, this is a loss for you and it is not your responsibility to take on the whole burden of their situation. Groundrules might be: they don't criticise each other to you, they don't ask for your opinion on the situation, they don't send messages through you but instead communicate directly, they maintain contact with your daughter and go along with the explanation you have given her for their separation. If you feel able to offer emotional support then great but I would make it clear that you are offering that to both of them and that you expect them to realise that you are not going to take sides.
For the record, mediation or Relate can help people to separate as well as possible. After making a real hash of their marriage, my parents had a very good divorce with both behaving in an exemplary and dignified way. I respected them enormously for that and it has made subsequent family events much easier for us all.
I wish you all the best. I hope you have good friends and a good partner to give you the support you need. It can be a shock, even as an adult.
Good luck.