OP, you make it sound like the only thing a child can do when in a car/waiting in hospital, is go on a mobile phone 
She could read, colour in, day dream, do an eye spy book, sing, play games with you, play with toys, use play doh....
If you want to buy her a phone, then do. But it doesn’t sound like you know anything about keeping kids safe online, so I feel you just want a mini tablet for her, get an iPod touch or something.
I really wouldn’t allow my 6 year old access to musical.ly (Or any part of the teen/adult internet) unsupervised.
WHen my dos was that age he had restricted access to apps like CBeebies or yahoo kids. He couldn’t download anything, time was limited, everything monitored, and what he could access was trusted safe content.
Having worked in schools where kids have fallen foul of being given adult gadgets And unrestricted access, my boy will not be exposed to dangers like that, just because “everybody else” is allowed to snapchat/play COD/Whatever