Depends where you are going Mummycan.
Durban you might be OK - I've certainly been swimming in the sea there at that time of year.
The cape might be a bit cool - it would just depnd on the waether - and it is not so reliable at that time (although we have always been lucky).
The pools may well be getting too cold to swim in, unless they are heated - it's nott he temeparatures during the day that is the problem, but the temperatures at night.
South Africans may have a different view as to whether or not it is warm enough. Dh and I have a fond recollection of sunning ourselves on the deck of our Rondavel in the Game Park on our honeymoon (beginning of June), me in my bikini and dh in hs trunks, when one of the rangers came across to see us - in her fisherman knit thick chunky jumper. We each looked at each other as if we were nuts!
Innorway - I'd prbably go for the 1 tog. If it were that hot, then you could just put a sheet over your youngest ones.
When we were in the Cape in November/December 04, it was really hot during the day. However, we all still slept under sheets and a blanget at night - so porbably about the equivelent of 1 tog.
I agree with jambot about not even being asked permission befire taking your kid away "to look after". The other thing is that they hate hearing kids cry and will do anything to stop them crying. This caused me a wee bot of a problem with ds when we were there when he was 13 months old: he used to resist having his afternoon nap, and I was at a "Golf/Activities Day". I put him inside the golf club to keep him away from all the activity and hopefully get him to settle. He would usually grizzle a wee bit and then go off - however,the staff kept on entertaining him to stop him crying and then feeding him bottles of milk and grapes (both of which were OK - he was mixed bottle and breast fed by that point - but not too much) - so he never did settle. He rewarded me with the most spectacular crap, which went everywhere over his buggy right in the middle of the auction/dinner that evening.
Definitely take some form of portable high chair for your two youngest: you can't rely on finding thme in all restaurants.
We didn't get a wonderful car seat in either of the two cars we hired when ds was 13 months - but at least he was old enough to be facing forward. They were just adequate - and we didn't get instructions as to how to fit them (and at that time, we were still using a rear facing seat at home, so didn't even know how to start putting in a forward facing seat).