The OP's DD WILL see less of her dad if he isn't popping in for a coffee a couple of times a week, or dinner once a week, or occasionally going to the cinema with her mum and her dad.
Most of these times are times when she would ONLY be seeing her mum if they stopped.
So it stands to reason that if they stop, she sees less if her dad.
That's not rocket science.
So, by stamping her feet about this, the Ex's new GF is going to be cutting down the amount of times in a week the OP's DD sees her dad.
Which is unfair on the little girl at the centre of this who has never known any differently.
If it is all she has ever known, then surely she is going to feel that SHE has done something to upset daddy and that's why he doesn't want to see HER so much.
No amount of explaining that her mum and dad aren't in a relationship any more is going to change that feeling - she was too young when they split up to know any different, they've never been 'together' on that way to her, yet suddenly daddy stops coming in for coffee etc.
I foresee a little girl trying to pull her daddy into the OP's house, in tears, asking him to read her a story / see her new toy / look at her picture / look at her certificate, and daddy saying no.
And that won't hurt a child?!
I can't understand how people think it won't.
And the DD DOES get 'family time' with her dad and his new GF, and by extension will get that with her new sibling, but that shouldn't mean that she shouldn't get the little bit of 'family time' with both her mum and dad together that she currently gets TOO.
It's a quick coffee at pick up / drop off, a dinner a week, and the odd meal out. Why is that wrong for the OP to wish that to continue, when the alternative is an upset child asking her why not?!
And WTF do you say when it stops and it's all she's ever known?
This new GF's jealousy IS going to hurt this little girl, AND limit the time she gets to spend with her father there. Selfish.