Can you please help me figure out what's a good amount to give these kids for their graduation?

I’m invited to a graduation party next week. I’m not close friends with the grandparents. The party is for two kids, one is graduating from high school and the other is graduating from 8th grade. The party is at a local beach, and I’m going solo, without my daughter. I can’t stay too long because I’m invited to another party in the early evening. ‘Tis the season for graduation parties.

I don’t know the kids at all and wouldn’t know them if I saw them on the street, nor would they know me. I got invited by the grandparents when I DM’s the grandma a photo I keep of the other grandkids from 20 years ago and she was so happy to receive the photo, she send me a digital copy of the invite.

I had two thoughts, one is 20 dollars for the kid graduating from 8th grade and 40 for the kid graduating from HS. The other thought is 30 dollars each to keep it simple.

I’m open to other thoughts or ideas. What do you think?