Shingles in Children - Is there a weakness in my family?

My son had shingles about 2 years ago when he was about 6/7 years old. My 11 year old daughter has just developed it a week ago but I thought it was quite unusual for children to get shingles? In both cases it caused just a small rash on the torso front and back in the chest/upper back area. For my son the main rash was on his chest with a couple of dots on his back, for my daughter it's the other way round but weirdly in almost the same place.

I have 2 other children much younger and now I am wondering if there is a weakness genetically that is causing shingles to appear in childhood in my children?

If you have shingles as a child, what are the chances that you will get it again later in life?

I don't really understand what causes the appearance, I have read that if you are run down or have a weakened immune system it can bring it on but I can't really understand why my 2 children have had it.

Any thoughts would be greatly appreciated.

Many thanks

Clair