How to select Parent Post when edit post from frontend

Hye,
I use Types and Formidable Pro (I know it better than CRED) to create form to edit custom post types from frontend.
I have a Parent Custom Type "Shop" and its child_posts "Offers".
On the form to edit "Offers", I would like to add a select input with all the "Shops", in order to choose which "Shop" will be the parent of the current "Offer".
I didn't see some API or explanation for this kind of thing about "Child_posts". I just found some code to display child_post but, as you see, my trouble is different. Thanks for your help and idea…