This is not a new idea, I have seen lots of these done in various ways...just google how to sew a Tshirt tote, and you find all kinds of projects. I like doing these because they are such a great way to recycle and everybody has an old Tshirt.
First thing to decide is how big you want the bag. Measure it out on the front of the Tshirt with a yardstick,(easy for the rowdies to use) and chalk, you may also want to measure and chalk the back. Sarah cut the front first,layed this piece down on the back and cut around it. Rowdies love to work with chalk, they love tools of all kinds ...so of course they want to do the cutting. If you have a rotary cutter introduce the grandkids to this handy dandy little item...they are amazed!!!
The straps are very easy as well, make them as long or short as you want by cutting strips from the Tshirt. When you have cut the strips of material place the right sides together and sew a seam. Turn the strap right side out, (a chop stick works really well for helping little hands work the strap back to the right side.) Sarah did a lot of the sewing of the straight seams on her bag, I sewed the straps on for her since it was a rather small area and had to be backstiched.
I am not going to go into detail on how to sew the bag, again you can google tons of ideas for sewing it up. The main thing I wanted to share was the way she chose to decorate it. We had a blast painting her hands in bright colors of acrylic paint and smacking handprints all over her tote, front and back! She then used dimensional paint to write her name on both sides of the bag. (We painted the Tshirt before we sewed it, it might be better to paint it after.)
If you have a favorite Tshirt with a cool logo just be sure you cut your bag to incorporate the logo, additional paint may be added...cause they have to paint! When Sarah wanted a second, smaller bag we cut pieces from other old Tshirts and sewed them together before we made our tote. For this bag we used an old sweatpant drawstring for the handle, you could also use shoelaces or ribbon for handles.
If the grandboys don't want to make a tote bag have them handprint a plain Tshirt they can wear, make a pillow to sleep on, or do a simple collage on construction paper. I do have a grandson who loves to sew and he has made several pillows, and silly creatures from old clothes.