They never shoot one another, but you see their names in the police blotter every week for domestic violence type stuff.  

Then have them call you so you can meet the police on the property.

I had a bad experience having 2 people sign a lease around 6 years ago.  10 months in there was high drama - couple was fighting, police were called, he kicked her out, she wanted to kick him out, etc. 

Tenant should provide a copy of the police report showing a crime was reported.

You can also call the police and ask for a wellness visit. 

Can I just call the police because there contract is over and they still are in my house, or do I have to do and eviction?

In the highly unlikely event that the local jurisdiction isn't familiar with the law (some law police are not familiar with the law), than any higher court that sees this case on appeal will GLADLY make sure you never make this same mistake again. 

The forum police are massively overhanded here so you will have to read between the lines.Here is the short of it.Travelers are sick of the fees and commissions being charged by the largest OTAs.

At what point do I call the police if there's a legitimate worry regarding their safety or the safety of their two kids?

I personally tell all my police officers friends to do this because it is an extra way to make money without doing all the work, just need to send the name, phone number, email address and if possible mailing address.