Suggestions for where to go: 10 days; start and finish Sydney

if your thinking of heading across to the New England, then consider doing the Old Grafton Rd which goes to Glen Innes, it really is a very pretty route to take. i recently suggested this drive to a Tasmania tourist on another forum who was up around those parts on holidays, after completing the drive he said it should be on everyones bucket list of drives to do. you can see some of the drive on this old trip report i did (ignore the first 3 days of the report) http://forum.themotleycrew.com.au/index.php?topic=865.0
if your into walking then you could maybe venture far enough North on the New England to visit Bald Rock or possibly even Girraween National Park also, it's hard work with a bub on your back, but the climbs can be done with them if you really want to do it, and the views from on top of the climbs make the hard work well worth the effort. here's another trip report i did from our visit to those areas you may be interested in (just scroll a few posts down the page to find it) http://forums.overlander.com.au/viewtopic.php?f=19&t=60924

Given you mentioned north along the coast, back along the New England with CT and the need to be relatively safe for a wandering 18 month old baby, BUT not knowing what you are "into" and amenity level, whilst assuming you have a 4WD, places I would consider are......
Pebbly Beach at southern end of Yuraygir NP or Sanden River(outside of school holidays) at northern end. Broken Head instead of Byron Bat itself

In New England region & at the back of Coffs Harbour: Dorrigo Rainforest Centre, Nymboida and surrounds (when it's not raining) I think are one of NSW hidden gems. Platypus Flat, Nymboida canoe centre if you are into kayaking. If you want to laze on a river for a couple days surrounded by valleys, National parks and Bass fishing, West Kunderang Camping on the Macleay River, approx. 1.5 hours east of Armidale, or for temperate (cool @ 1200m) rainforest Wattle Flat on Styx River 1 hour east of Armidale.

Just some thoughts for you to evaluate

Old Grafton Road is an overlooked gem but it is narrow, windy and some quite steep drop offs. That said the camp area at Dalmorton is a top spot. A good place to set up camp and explore. From there head south through the National Park to Dundarrubin and Dorrigo. No facilities. Check road condition with NPWS.

Mullaway just North of Woolgoolga is a great little spot. Caravan Park there. Overlooked by almost everyone. Danger is you may decide to go no further. One shop, no bottlo and a few houses. All the shopping and supplies can be bought in Woolgoolga (Woopi) which is a top spot in itself. Safe for the kids. Shop has best take away. Just do not expect fast service especially at holiday time. My suggestion is to stop, make your order, head to the beach and come back at a pre arranged time. Expect a LOT for your money. The sign says "We don't make fast food we make good food."

Red Rock is another overlooked spot. Van park on the river. Bowlo about 300m up the road. No through traffic ( like Mullaway) so safe for the kids. Small shop att the van park otherwise off to Woopi again.

In my opinion the best place to stay in Byron Bay is Brunswick Heads. Yep, I know they are two different places miles apart and that is my point. Byron is crowded and touristy. Brunswick is far more lay back, off the main road and relaxing. Ferry Reserve is a great park north of town past the boat harbour. River front. A bit of traffic noise but nice.

Best pub stays would be the Ocean View Hotel at Urunga, Ulmarra Hotel at Ulmarra and the Brunswick Heads Hotel at Brunswick Heads. All with water view, all have good food, clean accommodation and off the main highway away from the road noise. Ulmarra Swimming Pool is just next door to the pub. Ulmarra is a tiny little village on the highway near Grafton but worth the stop. Probably THE best beer garden in NSW.

Thanks guys for all your replies and views. I'll def give Dalmorton a visit I recogn - sounds like a great drive too!

Ps yes we've got a 4wd and off road camper so keen to get off the beaten track