I'm thinking of contacting my niece's kids school to alert them about my niece not taking them to school regularly
What is going on?
They need to know that kids are skipping school.
This sounds like an extremely difficult thing for you. Instead of immediately alerting the school, however, you should talk to their mother first and try to resolve this without getting them in too much trouble.
I don't have time for this.
You're right that this isn't exactly your problem, but they are your family members and people will expect that you at least do the minimum in trying to fix this problem. Speak with their mother and see if she knows about them skipping school, because odds are she doesn't.