I misunderstood you a bit Fiz and was thinking you might want to get to Sydney a lot quicker but if you want to spend nearly a week ( and this is during January isn't it? ) , I'll offer a bit more for you to consider.
First off, the coastal strip will be pretty busy during January ( it major school and also employees annual leave time of year and so accommodation around beach places will be heavily booked/sought and traffic more of a hastle ). Even away from school holidays the motorway from Noosa/Sunshine Coast down past Brisbane via the Gateway ( Toll bridge over the Brisbane River ) towards the Gold Coast and beyond is something of a multi lane drag strip with upgrade roadworks being a pain at times.
Kingscliff is a bit of a drive away from the Pacific Highway and not a town that I would say has anything too much of significance to warrant diverting to there though I do kind of remember there might be an aaarea where Koalas hang out ( not sure whether it is a sanctuary or not ) and back over towards the highway in that neck of the woods there is a Tropical Fruit World where you'll find about 50 or so varieties of tropical fruit. The beach there is not so great from what I remember, it rather exposed and wind blown and if I remember corectly I think it was in the news a year or so ago re a lot of storm erosion that was gobbling up the foreshore including the local caravan park.
As you move further on south past Byron Bay, you'll find that the motorway if anything ( like all motorways do ) makes for a kind of nondescript and even boring drive interspersed with sections where there's not yet motorway and so traffic banks up - http://www.rms.nsw.gov.au/documents/projects/pacific-highway/pacific-highway-rest-area-driver-reviver-map.pdf is an indication of where there's motorway and where it is still being constructed.
So, with all that said, if you want to do something different for a few days away from getting to beach areas and seeing something of the hinterland, you can get to see some interesting countryside and you will have far less traffic including bypassing the drag strip down past Brisbane and the Gold Coast.
Into travelmate.com.au, plot Noosa - Esk - Boonah - Kyogle - Nimbin - Nimbin being the unofficial capital of NSW northern rivers alternative life style ( a bit of pot about that neck of the woods ), but a pretty harmless and friendly hippiville kind of place.
You could probably get a motel for accomodation back at Kyogle and that will be on your way further south.
From Kyogle, you could either head up to Tenterfield on the New England Highway or continue on down to Grafton and then plot Armidale into Travelmate and that will take you up along part of what is called Waterfall Way - http://www.nationalparks.nsw.gov.au/things-to-do/driving-routes/waterfall-way-scenic-drive - you would join the Waterfall Way just before Ebor and the falls there.
It is all going to be a bit winding in some places so if anyone does suffer from car sickness, it is probably not for your family but other than that there is some delightful countryside to travel through and sure beats the hell out of being in mass traffic on the Pacific Highway.
Armidale is an inland University city that also services a lot of rich farming, there being a few wineries around too, quite a reasonable place to have another stop.
You would have two options from Armidale:
1. Stay on the New England Highway all the way down into the Hunter Valley, you passing through another large country city called Tamworth - Country Musical Capital of Australia ( and a small bypass there to miss city centre ) and down around a place called Scone you get into horse stud country but a bit further on there is the eye sores of a lot of coal mining being done.
You could turn of at a place called Muswellbrook for a place called Denman to miss the worst of the eye sore and then in heading east from Denman, take a southerly turn on to what is called the Putty Road which takes you through the Wollemi National Park to bring you to Windsor which was one of the earliest inland settlements near Sydney, it actually having the Australian mainland oldest continually operating hotel.
The other Option:
2. Heading south from Armidale, about 25 km. south at Urunga you can turn off the New England Highway for Walcha and then follow Thunderbolts Way down to Gloucester and then Bucketts Way back to the Pacific Highway to join it about 30 km. north of Newcastle, there being some captivating inland ranges views along the way, one in particular having a pull off to stop for having a look.
It is a different way to go, away from the hurly burly of the Pacific Highway and far more relaxing.
If you took the second option, you could head into Port Stephens for a beach dip at Nelson Bay and stay thereabouts overnight and then maybe do some sand duning, something the kids will likely enjoy.
If you stick with the coastal route, I;d forget about Kingscliff and make it to Byron Bay for a stop or for something a bit quieter, try Lennox Head, just a little further along and maybe easier to get accomodation if you are in peak January time.
The extra day, You could add to South West Rocks or perhaps substitute Nambucca Heads for SWR, there being a great little holiday village just north of NHS - http://vallabeachtouristpark.com.au/ - great spot for kids, very family orientated with cabins, a play ground and all sorts of fun things to do.
If you want an extra day for Sydney ( and plenty of things to do there too ), you might want to skip Forster.