Hey

Unfortunately the 777-300 is not the ER. However, DXB-KIX is on the A340-500, which is sure to be good.

The seats seem to be ok, nothing great, but nothing bad. Depending on the age of your kids - you might be able to pre hold the bulkhead seats which are much better than the standard ones (especially on the A340-500 which only has a 32" pitch or so vs 34" on the 777)

Another comment -

If you would like to be more comfortable for the JNB-DXB sector, there is an A340-300 operated flight as well (the afternoon one) - the seats will be much wider, and the plane newer, although with slightly less seat pitch. Not worth paying a change fee, but if your ticket allows free changes - it would be more comfortable. I am assuming that you are stopping over in DXB as the afternoon flight from JNB (on the A340-300) is the one that connects on to KIX directly

Cheers