Your father is very fortunate to have such caring children! You are correct, it is not taxable income to you. However, the amounts that were paid can be claimed on your father's tax return if they were for qualified medical expenses on Schedule A.
No, no reason to report this financial event. You loaned money to your father and he paid it back (kudos to you for doing that). However, the interest earned must be reported on your tax return.
The IRS requires that adequate interest is charged. Read more about it here.
You probably want to use the applicable federal rate (AFR) to calculate what the adequate interest for the period should have been and include that on your return. You can find the rates here.