We were in the Karoo on a friend's farm this summer. I took my 5 year old out for one hunt. He was absolutely gutted that we didn't take him along in the afternoon. He wouldn't of coped with the distance and heat though.
I took him out round a local farm in Cornwall last winter. I was surprised at how long he kept going for, as it was bitterly cold. He also managed to stay relatively quiet.
I think if you take a youngster along then enjoy it, have no pressure on you or them and no great expectations as to their behavior being perfect.
He's still keen to come out especially as his Dad hasn't produced an eland or a deer in his company yet.