Oh dear, I know how you feel. Especially with your poor Dad being 90, it is so hard. For them it is the start of a big adventure, you just have to be supportive, and not let them know how you feel inside.
My son went down under on a 2 year working visa, that was eight years ago and he is still there! He did come back last year for a year, but now returned. We have only visited twice, due to money problems,.
We have a family WhatsApp group, all free, so we can easily stay in touch. If you Dad has a smart phone, he could be on it too, and it's so lovely getting little messages and photos of their latest travels. 