Look these things only you can really say what your family would like; for a long weekend I might still do Durban but only if I could leave the Thursday and even then that last day becomes tricky - ideally you want to leave early to avoid queues heading out but then you lose that day. So get there Thursday late even, and you've got Friday and Saturday, leave early Sunday. Definitely doable especially with kids at that age, they're old enough to keep themselves busy.
And then there'll be plenty there to keep you guys happy.
Sun City or even WarmBaths, they're nice cos the 14 year old especially can do their own thing for a bit; he's probably getting to that age where he wants to do some stuff on his own. Plus both within like 2 1/2 hours away.
But I mean 5 hours from JHB puts you in many places in SA, heck, you can get to Kruger in that time or places like Graskop, do the whole God's Window, Bourke's Luck Potholes scene. I'd almost say for that period a long weekend is too short though - too much to do but you could pick what looks cool.
Plenty options I reckon.