Password leads to one of multiple password protected sites

Hi there,

the title is pretty confusing, so I try to be as clear as possible with my intention.
I’m setting up a photography portfolio where I have the possibility to add password protected gallery sites for my customers.

What I like to have: My customers have to enter my site first if they like to access their gallery, so I will not send them a direct link to the gallery site, so that they just need to know their password for the protected page. They always need to access the root site https://www.mysite.com.

I’d like to add a parent site to my menu (for example: customer login). So far: all right!
And now the tricky part:

The parent site (let’s call it “/galleries” has multiple password protected child sites, which are the various galleries, example:

PARENT: galleries
CHILD: customer 1 (password protected)
CHILD: customer 2 (password protected)
CHILD: customer 3 (password protected)
…
CHILD: customer 14 (password protected)

and so on.

What the site /galleries has to display ist just a password field, no user name, no registration or anything like that. No matter if it’s just a php script or a plugin to put there.
The typed password now has to check all the child sites and has to open the one who fits to the typed password.

I know it’s not a secure way but it perfectly fits to my needs.
So my question: is there a possibility to get this done?

Thanx in advance!