Personally, I will recommend an all hardware solution. However, for your parents in Kiev to call your grandmother in LA using a computer, and the telephone network will still require some set up. I am not sure if your parents will be able to (as it may only support US - US/CA calls), however they might be able to use Google Talk. In which case, they can use Google Talk through a Gmail interface, or download Google Talk on their PC, and download any plugins. They will need a Gmail account.
The second option is to get a SIP option. Local Phone offers calling to the US for $0.009 (9/10th of a penny) to US/CA/UK calls. This will allow them to keep calls on their end cheap, and they can even get a US telephone number which will allow your grandparents to call them (at her rates).Your parents should be able to get an LA number for $3 setup, and $3/month at last time I used them. They do have a Windows software client that should just require logging in. They can use other SIP clients, but this will require setting up credentials.
Personally, I will suggest a SIP phone as this will demand less on the computer, and if there is a wifi or wired network, it will work if there is something wrong with the PC. If your parents have an Android or iOS device, they can install a SIP client, and make the calls on the phone they are already used too.If you want a flat rate, the cheapest option (although will violate the TOS) is to set up a Google Voice account with a LA number, get TelTub ($60/year) and an ATA ($40) which will interconnect with the Google Voice account, and therefore outgoing calls will be free in the US/CA, and actually show a telephone number on your grandmother's CID. GV only offers Google Voice to US customers, so they may block foreign IP address. A second option will be Nettalk which is a hardware that will plug into a router, and works in most other ways like a Magic Jack.
Your parents will need at least 128kbps both ways, and less than 100ms for SIP to work. Have them go to speedtest.net for verification.