I would not raise the rent... Keep it the same.
You would be crazy to raise the rent. They are doing property management, maintaining the property, maintaining the lawn, garden, etc. A property manager will charge you around 10% of your rental income to do everything you stated. If they are willing to do all that work... Nothing is to say they will not be ready to move out and buy their own place.
Would you really want to take on the headache of pissing off the current tenants and assuming property management? Sounds like everything is working out exceptionally well. At this point raising the rent just sounds like greed that can easily backfire.