DD1 just accepts that we aren't married, it isn't really something she's asked about TBH. And we were both in my sister's wedding party last year, so it's certainly been up for discussion. She just thinks it's funny that I have a different last name, but since at school I often get referred to by their surname it doesn't bother her. When we do get married, I'll keep my surname anyway.

We do tell her we might get married one day, we'll see.

And kids kinda assume you love each other, they accept the version of family they see before them as normal. You don't need to even mention it. In some families, mum and dad get married first then have kids. Some families do it the other way around, that's all. And some don't need or want to get married, they're still a family.