Partner with a Daughter: Need Some Advice!

Hi ladies (& gents possibly!)

I’m 27 and I’ve begun dating a wonderful man who is 33 and has partial custody & care of his funny, smart and sweet 12 yr old daughter.

I enjoy spending time with both of them, separately and together and we get along well! The only thing I’m having trouble with is - being the relatively new girlfriend, sometimes I see and know that the daughter needs to be encouraged or told to do or not do things when I watch her by myself, but I’m having a hard time helping out my partner with that. I have a background as a nanny so I’m no stranger to nurturing kids but I don’t want to step on toes or make the daughter resent me? I don’t feel I’ve earned the right (?) to help act as a parent yet... Also sometimes they disagree about little things in front of me and kind of put me in the middle which makes me uncomfortable!

Basically I love my partner, and his lovely little daughter, but I’ve never been a mother and I don’t know how to navigate and grow this relationship with his daughter. Any helpful, experiential advice would be so appreciated! Thank you.