I was a teacher (2nd grade the youngest)
Google "Brown paper bag scrapbooks" and you can see a really cheap and easy way to use paper bags to make a little book. Or just click here. http://www.scrapbook-crazy.com/paper-bag-album-instructio... (This is the best tutorial, IMO b/c it shows pictures of the steps. Just scroll down past the Ebay stuff and the ads).
For first graders, make a template for each page. IE: each kiddo gets two brown paper bags. Perhaps you could have them pre-folded and pre-whole-punched. Depends on if you have adult helpers. Each bag will need either two or three holes punched to keep the 'bind' of the book together. Yarn will work, but ribbon looks prettier.
COVER: have a premade title or let them choose a title to write/copy from the board.
INSIDE pg1: I like it when my mom _______.
INSIDE pg 2: My mom is as pretty as a _________.
INSIDE pg 3: My mom makes me laugh when she ______.
INSIDE pg 4: My dad thinks my mom _____________.
INSIDE pg 5: I like it best when mom cooks _________.
INSIDE pg 6: Here is a picture of me and my mom.
BACK: Happy Mother's Day. I love you, Mom. Love, _____
just an idea of phrases. It won't take as long if you have them pre-printed where the kid fills in the blank. I don't think their hands are small enough, but on the back page, they could put their handprint on it with paint, or trace their hands.
ps. Don't forget that some kiddos don't have a mom at home. So if you do templates, have enough ready that say "grandma" or "aunt" or "neighbor" or just a blank space so they could write the name of a female caregiver. ie: "I love it when ____ cooks _______."