Well, these were two high school girls in their student's uniforms, and you know how some schools take students to see the firefighters' headquarters as a field trip, so... even though they are a little too old for that, I'd guess they'd be used to seeing school students around.
I dont know about that casually but i think its possable to go and ask to look around. It is technically a public service/place. You would have to ask out of politeness and to not get in the way of their work. Kids get to do feild trips to places like that all the time and im sure they have thankfull people drop by just to say thanks sometimes or highschool kids looking into how to get a job with them after they graduate coming by. Its probably like going to the sheriff offices or court house, you only go if you have a reason to visit and are not disrupting their work and they probably have rules about what areas/things your allowed around or to touch.
Can someone just go inside firefighter's house (?) just to give something?
She is not even related to him.....
I dont know maybe japan has that kind security.
All in all,it was unique and cuteeeヾ(❀╹◡╹)ﾉ~