I am a Canadian citizen and a permanent resident of the US. Would I be allowed entry into Canada to visit my elderly and ailing father during COVID-19 border restrictions?
Of course you can go to Canada; they can't refuse a citizen entry, not even for a pandemic. You will need to be able to prove your Canadian citizenship, ideally with your Canadian passport. You will be required to self-isolate for 14 days or be assigned to a place where you will be quarantined, if you do not have a place to self-isolate.
It is important to note that, during this quarantine period, you may not have any contact with "vulnerable people, such as people 65 years or older, or people with underlying medical conditions."
You also need to bring your green card so that you can return to the US.