My sister lived in Frankston until about two years ago, maybe less. I've never noticed any trouble when in the area. You get louts in most areas! However, I'd choose the southern end over Frankston North (The Pines/Kananook Creek area is where the Frankston stereotype comes from). It's not like you're going to be hanging about the train station at night though...

It's got great public transport, beach, shops, etc...

Also look at the suburbs Emma1979 mentioned and also Seaford, Carrum, even Patterson Lakes (more inland).

I'd love to live near the beach!