Terrie, what a difficult decision since I know that you love all of the kiddos that God has placed in your life. The nice thing is that they know you love them, too!
That said, I agree with a few of the others that said graduating from HS is a much bigger thing that from 8th grade. In truth, I really didn't even realize that 8th grade graduations are celebrated anymore! We've never been invited to one. Ever! (But have been invited to a few nephews Kindergarten graduations. Go figure!)
So, if I was in your place, and especially since you'll be going through this again in the future, I'd set the precedent of attending the HS graduation, but finding a special way to acknowledge the 8th grade graduates, too.
Maybe you and your sweet hubby could take the graduate out for a meal and make it an "event." (examples- make it a 'dress up event' where you go to a nicer restaurant, let them order anything they want from the menu, or present them with a special gift to remember the event- jewelry, a phone case, etc.) When I was a kid I remember that a friend's favorite aunt took her out to eat at a fancy place to recognize an event (might have even been 8th grade graduation... I can't remember that far back!!) She used the event to teach her niece basic dining etiquette in a fun and creative way. My friend was so excited to tell me how much they laughed because part of the "lesson" was done while talking in funny English accents, drinking tea from fine china (with their pinky finger raised), all while gently dabbing their mouths with the napkins! A few years ago I was the featured speaker at a mother-daughter event at our church, and remembered her story. I did some research on high tea etiquette and built my talk around it. I still remember the giggles that erupted as they practiced pouring tea... and learned the do's and don'ts of manners. (The really little ones loved when I told them one thing they must NEVER do if they needed to leave the table to... um... visit the bathroom was to announce to the table "I have to pee" or "I have to poop." Instead, they practiced politely saying, "Please excuse me." ) A few parents thanked me afterward, especially since I noted that talking with food in your mouth was a "no-no" (because it was just gross!) and using a cell phone or watching TV while sharing a special meal was also a "no-no" because it made the other person/people feel like they weren't important. They said that teaching manners was a tough task as a parent because so few people followed them anymore. Sadly, I had to agree. Those of us who are adults still need reminders from time to time, too.
A special time just with them would be something an 8th grader will remember more than if you were there to watch them "graduate". Just my suggestion.
Kathy