Noise Complaints in a Multifamily

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Hi, I know this is a tired topic, but I honestly don't know how to handle this since I'm not at the property myself and it isn't a situation where the police have been involved. 

Tenant 1 is complaining about the noise of Tenant 2. They are both good, long term tenants who pay their rent and take care of the property. Tenant 1 has spoken with Tenant 2 multiple times about the problem. I believe both sides are trying to be amiable about it.  Tenant 2 has two very young kids who are the source of the noise.  I don't think the family with kids is ignoring the complaints. I'm sure they are trying to keep the noise under control, but it keeps coming up because young kids can be loud and unpredictable. I have reminded all tenants about the "peaceful enjoyment" portion of the lease.  Tenant 1 is on a 1 year lease. Tenant 2 (the noisy one) is month to month.  

I don't see how I can take action against one tenant based just on the word of another tenant, even if I believe the complaint is most likely valid. I am grateful there has been no police involvement, but at least that would give me cause to give a warning, or to terminate a lease. 

So, should I just throw up my hands and tell them to work it out? Any ideas?