This is a wonderful idea, Tami.
(I also have school aged children). Oldest in 4th grade.
Here's a quick list that I found, that may be known by this age group, of those that eat a vegan or vegetarian diet.. and it may be better to word it like that, as this list seems to have a variety of folks to that do eat either way for a variety of reasons (ethical, dietary, etc.) which may be a help to you, when listing the reasons why people may choose to adapt a vegan lifestyle or eat vegetarian.
Leonard Da Vinci
Avril Lavigne
Mayim Bialik (the girl character from 'Blossom' who is also incredibly intelligent! A nueroscientist - and has done voice work for The Adventures of Johnny Quest, Johnny Bravo, Recess, Kim Possible.. - same for Alyssa Milano from 'Who's the Boss' .. believe both of those shows may still be in syndication, so a possibility kids in that age group have heard of the shows (at the least) and may remember the 'girl' characters (now women, of course!)
Ellen Degeneres (my daughter thinks she's hilarious - ask your daughter, if she knows who she is.. )
Casey Kasem (the voice of Shaggy from Scooby Doo)
Tobey Maguire (didn't he play Spiderman or another superhero??)
Jason Mraz (singer/songwriter - the guy who sings 'I'm Yours' on the radio..)
Pat Neshek (a MLB player)
Natalie Portman, Padme from the Star Wars trilogy, Mr. Magorium's Wonder Emporium, and read that she traveled to Rwanda with Jack Hanna in 2007 to film the documentary 'Gorillas on the Brink'
Ruben Studdard (from American Idol)
Former President Clinton (who does say he still eats a little fish, on occasion)
Jaci Velasquez (a popular Christian pop singer)
Albert Einstein
Anne Hathaway (which is a no brainer for your list, as she's been in Alice in Wonderland, The Princess Diaries, Devil Wears Prada, Ella Enchanted.. the list goes on and on). It reads that she also works with charities like the St. Judes Children's Hospital and The Lollipop Theatre Network that screens films for critically ill children.
Hope this is some help! Would love hear how the presentation goes.
To be honest, her classmates behavior at lunch, reminds me of a few interactions with ADULT relatives. Hmmmm.. ;)