I would tend to say
"[X] happens so frequently it is now a part of girls' daily lives." (without 'us' or 'we').
But if those girls your refer to are your daughters, you can say:
"[X] happens so frequently it is now a part of our girls' daily lives." (then we use 'ours' because it's possesive, the girls are your daughters)
As to why you can't use 'we' or 'us', I can't actually explain that, sorry... It just feels more natural to me. Since I'm not a native English speaker, I wouldn't risk trying to give you an explanation either because it might confuse you. So, I just leave that to the experts. Hope that this has been helpful to you, though