Birthday Presents for step daughter?
dolls lego pencils and textas
How about asking the parent she lives with, who would know what her interests are? You see her three times a month and you don't know her interests? I have nieces and nephews I see much less than that, some I even only see at most once a year, and I can still tell you what they like.
How can you and your spouse not know what you daughter enjoys? Don't you talk on the phone with her when she isn't with you? Don't you interact with her when you are together? She would probably like best some "quality" time with her instead of whatever you are doing now....
Since there's not much you know about her, this can be tricky. Definitely don't get her clothes/food. Perhaps get her a toy. If you could, get in touch with her guardian and ask for opinions. If not, getting her animal stuffed toys like build a bear would be safest.
A simple barbie with maybe a camper van or a small house. Maybe some makeup, like from claire's? Their makeup is weak, so it really doesn't show a lot of color.
Ask her mother
a kkk costume
We don't even know what her interests are
I suppose asking the custodial parent is out of the question? Oh and protip a 7 year old is neither a toddler nor a pre-schooler