You need to follow the N12 highway which joins the N4 at Witbank.
This route is well described earlier in this thread (the 1st post I think).
From Randburg you need to get onto the N1 and at the N3 split, take the N3 south.
At Gillooleys interchange, take the N12/R24 east split and move into the right hand lanes as soon as you can as the highway splits almost immediately into R24(to the airport) and the N12 to Boksburg, Benoni, Witbank. If you end up on the R24, don't panic.
When you get close the airport follow the signs to the R21 south and then onto the N12 east/ Witbank.
Its only slightly longer.
Time needed to get to KNP: Malelane gate will take about four and a half hours.
We like to stop at Middelburg Utra city or Millys (both are petrol stations with fast food joints)
Middelburg is closer to Jhb.
Choice of stop depends on the kids toilet needs.
We also like to stop at Erics farm stall which is in the lowveld (ie once you are down the passes) about 1/2hr-45 mins from Nelspruit.
They make great pancakes there.
Crocodile Bridge gate is about 40 mins further than Malelane gate)
The drive to Lower Sabie can be done in an Hour but will usually take longer as this is a great road for sightings.
If you go in at Malelane gate the drive is much longer to Lower Sabie (could be as long as 4 hours),
Another option is Kruger gate, also about four and half hours from JHB and about two and half hours drive to lower Sabie, once again remembering that this is another great sighting road.
All in all, my 1st choice gate with younger kids would be Croc Bridge.
Without kids,
I would probably go in at Kruger gate
Have a great trip