I think it's a fairly recent tradition for the royals to have mostly children as attendants at their weddings. By fairly recent I mean since Princess Margaret's in 1960. Kate and William's was a little unusual in that, apart from Pippa all of the attendants were from William's side of the family.
It's also the custom in Australia to have child flower girls as well as adult bridesmaids. We've tended to follow the British in this, as apparently have Canadians. There are lots of outdoor weddings here, as we have the climate for it.
I've been to weddings where children have been both guests and flower girls and boys and where they've been absent. (Sometimes, nowadays, attendants are the children of the bride and groom!)
I have to say though that here they do add to the joy of the day, when it is daytime. I've only ever been to one wedding reception after 6pm where under tens have been present. The little girl concerned was very young and went to sleep in a back room after a while but didn't cry beforehand. Most of the weddings have included relatives' children and why shouldn't they be part of the celebrations? They are part of the family.
As far as Royal weddings are concerned I wouldn't be surprised if these children have nannies who whisk them away before the wedding breakfast, when they start to become tired and whiney. If George is present as a pageboy at Pippa's I'd say that's what will happen with him.
The little flower girl at Kate and William's wedding who looked disgruntled was Grace van Cutsem. I believe she was only three or four at the time and I think the sound of the crowd below when she was on the balcony with the wedding party hurt her ears.