I think it's very easy to make a snap decision when you've had family visiting for a while. I've been overseas for over 7 years now and certainly in recent years when we've lost family members and I've not been there to attend funerals or weddings of my close cousins, it does make me feel really sad and on particularly drunken occasions I'm minded to blurt out "lets move back to the UK and be close to family!" And certainly when we drop family at the airport, I get very upset.
But then I'm reminded of how amazing our life is here, how our small kids are happy and thriving, and how when I speak to my parents about moving back, they immediately say "don't do it!" - they know when I'm wearing the rose tinted glasses (usually when I see snow or when I'm craving a Gregg's steak bake, or watching Gavin & Stacy) and they bring me back down.
For us, moving back to the UK would only be the better option if we had reasons other than family to cling to, even though we have a wonderfully supportive and close family. As other friends who have moved back to their home countries have found, life has moved on and the big family gatherings are not as frequent as they had imagined.
I try to travel back to the UK every year-18 months and my family always throw a big gathering (huge family, logistical nightmare to arrange). Aside from my visit and unless there is a wedding or funeral, there don't tend to be as many as I imagine there to be.
Australia, in terms of opportunities for your children, is great. You and your DH both have a wonderful life. I think the fact that your parents have just left is clouding your judgement a lot (as it does mine when my family leave) so I would sleep on it for a while and then if it's still bugging you, approach it with fresh eyes.
But whilst I agree that having family close is wonderful, my sons don't seem to have suffered from not physically seeing their grandparents and cousins very often. Skype is your friend here - I skype my parents every other day, my 3 year old chats with his grandparents all the time, 10 month old recognises their voices, and I've no doubt as they grow up, they will feel just as close to their grandparents as I did to mine whom I saw very regularly.