I've heard of airlines having rules about the ratios of very small children to adults--for instance, lap brats must be held by adults and not older children--but AFAIK you can bring as many school aged kids on a plane as you can stand or afford. I was just amused by the statement that they will always sit kids under 14 next to a parent when each parent only has two side. In narrow body 3-3 seating, when you put the parent in the middle seat, this allows for a family of two parents and at most four kids, which isn't an extraordinarily large family.