The answer is a very personal one and depends on many things. IMO $100.00 is a very generous gift for a family member graduating from high school.
My brother, who is very well off financially, gifts very, shall we say, conservatively. Other family members seem generous to a fault, beyond their means.
The monetary amount of your gift can not define you and your husband, nor can it define your relationships with the graduates.
I would save the larger gift amounts for college graduations, when those loans start coming due!
I am sure those special graduates will genuinely appreciate any amount they receive.