Here's our set-up, for 2 adults and a child. I can't see how you'd go for less space and still have everything you need.
My pack: WE Karajini (90lt)
Wife's Pack: Mont Pioneer (75lt)
Laura (Aged 6) pack: Kathmandu day pack
Tent:
Coleman Extreme Phad (which is for sale) OR
Hilleberg Nallo 4 GT
NOTE: both of these tents have vestibules, which is important IMHO when hiking with kids
Sleeping mats: Exped Downmat 9 (too heavy, but soooooooo nice) I'd go for a prolite if I was buying it all again.
Sleeping bags- We had to cut corners here due to a) cost and b) the worry that liquids would get into down bags.
Me: The North Face Mammoth (I'm hefty)
Wife: Mountain Hardwear Lamina 20 (an outstanding bag, and I'd take one over a down bag. Also, at little over 1 kg, I wish I'd gotten one for the kid too)
Kid: TNF Blue Ridge (fits to 5 foot/1.5 metres, good to -7 with a thermal insert and clothes on)
Stove: I got a kovea moonwalker, which is heavy, but stable. I was going to get an ultralight titanium job, but both the guy in the store and my wife shot me down, and they felt these were too unstable for using around kids.
Here's my experience with the stuff you've listed:
I-Explore tent (to fit all three of us) TOO SMALL! FOR 3 PEOPLE YOU NEED A FOUR MAN TENT. STUPID, I KNOW.
OP Strezlecki 80L (which will be the pack I will be using to carry all the stuff) If the family not liking the hiking bit, I can still go ahead myself. Intend to work towards multiday hike such as OT, etc. NOT MY CUP OF TEA, BUT THEN I'M DIFFERENT TO THE HORDES THAT LOVE OP PACKS
Camelbak Helena 20L (which my wife will carry, and it is also the daypack that she uses for our normal bushwalking) WAYYYYYY TOO SMALL- REMEMBER- KIDS NEED 2-3 TIMES THE CLOTHES YOU NEED, AND IN THE END THEIR GEAR ENDS UP WEIGHING NEARLY AS MUCH AS YOURS. WHEN I CARRY ONLY MY STUFF AND MY DAUGHTERS, MY PACK WEIGHS IN AROUND 21KG.
OP Bushlite 700 x2 (for the adults) NO COMMENT, OTHER THAN THEY ARE NOT WATERPROOF, AND THAT CAN BE A PROBLEM WITH KIDS (OR ARE THEY?) AND YOU CAN GET STUFF WAY CHEAPER FROM THE US
OP Sac 2 (for my daughter) something she could also use for her school camps down the track.
Thermarest Prolite x3 (GOOD CALL)
Camelbak bladder 3L x2
Usual clothing (ie rainwear, warmwear, etc)
Maybe the rocket pocket (not sure it is suitable for us) (DEFINITELY NOT STABLE ENOUGH FOR USE AROUND A KID)
Food (KIDS LIKE THIS

)
Any other things, that are not coming to my mind right now.
Do you think I can fit all our stuff between my wife's daypack and the OP pack? Will it be too heavy for me if i could fit it in the pack? NO AND YES
Also, any ideas where we can start our camping to test out our gear? I am located at Burwood East, Vic. Thanks in advance. TAWONGA HUTS IN THE BOGONG HIGH PLAINS OUTSTANDING HIKE AND HUTS FOR SHELTERV IF YOU NEED THEM. LOTS OF BRUMBIES TO SEE TOO.
If I had to do it all over again I'd buy Montbell ultralight sleeping bags and prolite mats and keep the rest of the gear.
Let me know if I can help more.