We just had a daughter graduate from high school. In general, close family gave her $100 or a special gift that had something to do with her. Friends and her cousins (who are in their twenties) gave her $50.00 gift cards to Bed, Bath and Beyond or cash in a similar amount. Gift cards for gas (Wawa), supplies for school (Target, Macy's, etc.), or fun things like Starbucks are always appreciated. If your cousin is getting a computer for graduation, you might consider something for that - the carrying case if it's a laptop, a wireless mouse, software, etc.
For the younger child, if you're giving cash, I would give $25.00. But they LOVE iTunes gift cards because, of course, it's the age for figuring out music tastes. I also have made up a high school survival kit. Stuff to decorate the locker if it's a girl, not sure if it's a boy, but you could ask the parents.