Hi,
I have over 20 years experience in programming with more than 15 years in web development and quite some years with mobile apps. I can implement Your ideas fast & reasonably priced.
As far as I understand, in Your app people can sign up and then configure who else would get some notification about their location when they hit the panic button. And the app will also play some sound to alarm other people around.
So you will need some backend to the app also. I would implement that in PHP or nodeJS - however You want it. For the app itself I will use PhoneGap/Cordova & Ionic to make sure the same code can be used with iOS and Android.
Please contact me to speak about the precise list of features, ...
I do the "agile devlopment" approach - meaning, that through the entire process I update you with new versions, as soon as it has some major changes / new functions. You can than check the progress and we can talk about how you like it and what has to be changed.
I would start with a MockUp - setting up the user interface for the app, every button, ... will be there but with dummy data and clicking buttons will not do anything (in most cases).
Then I will make this app alive by adding functionality & the backend.
If you have any questions - just ask.
Best regards,
Fabian