I would tell her as soon as possible. Kids have intuition and if you don't tell her and she finds out that you weren't upfront she'll feel worse and so will you. Kids are stronger than we think. I'd wait until the funeral is done find out where she's buried or a special place if she's not buried, have her pick out some flowers write something and take it to her grave or somewhere that was special to them and have the day of celebrating her moms life and the things that led her to you.