I would make it a positive thing.
Perhaps talk about how you aren’t the mummy that grew her but you’re the mummy who
chose her because she was so special.
Be prepared to answer a lot of questions about her biological mum though and try not to let her curiosity upset you. She’ll be fascinated with the idea of having another mummy out there someone. You’re going to have handle it very sensitively yet honestly.
My DD’s dad left me when I was pregnant for another woman and has had nothing to do with either of us since.
I’m very honest with my daughter about it.
She asked me once why he didn’t want her.
My response was “It’s not that he didn’t want you, he wasn’t grown up enough for the responsibility of raising a child. Everyone that meets you thinks you’re amazing and loves you, he’s just never gave himself the chance to fall in love with you”.
I also manage her expectations when she asks if she decides to find him when he’s older would he want a relationship with her.
I’m honest and say I don’t know, not everybody behaves how they should and he has a different life now... one where he might not have a space for her.
She’s ok with it because we talk openly about it whenever she wants to. There doesn’t appear to be any hurt there for my DD because I think I’ve handled it appropriately.
I also say “Sweetheart, he can’t miss what he doesn’t know”.
It’s hard because you don’t want your child to feel rejected by the absent parent. I try to emphasise how loved she is by everyone in her life, it’s not about her... it was about him and his issues.
And basically I have a very balanced, happy, emotionally and academically intelligent daughter.
I also let her know how lucky I feel I am to have her.
I think you’re doing the right thing by telling her whilst she’s young. Younger kids are so accepting of what they are told and if you do it carefully there doesn’t need to be any long lasting damage to her self esteem.
Good luck 