The Catholic tradition is for the bride and her guests to stand on the left side and the groom and his guests on the right side. This is not a rule and in the cases where there are more guests from one of the sides, it’s better to have a balance and indicate a place instead of a side.
Usually between 4 and 7 years-old. Children under 4 years-old may not take instructions well or deal well with dozens of people looking at them. Children over 7 years-old may feel too grown-up for the job. But of course, if you know the little ones well and know they’ll do a great job, go ahead!