When my sons were in preschool and kindergarten, they wore a navy colored vest that I designed for Gotcha Toddlers. It had 2 thin stripes across the chest. It was great for fall and spring and looked great with their uniform khaki pants and white polo top or button down colored poplin shirt. It is a classic that you can use for church and for girls as well. Here is a cute vest pattern I found from AG Handmades for 99 cents., a free striped pattern in Russian from babyblog.ru and solid v neck one from Lion Brand for boys. I suggest a cotton in school colors or a sport weight acrylic. I like a commercial look not a handmade look. Keep the v neck high but big enough for the head and the length to be to mid or high hip. The shoulder seam should be wide, A rib hem in same main color looks clean.
This is a cute pencil case that is easy to make. If you like to crochet, why would you buy one at the store? They are probably made in China by slave labor. This would make a cute girl scouts project, too. This one is from Crochet Spot.
A crochet backpack is nice but remember that the books they carry are very very heavy. I would line the straps and the bag with a thin woven cotton/poly strong fabric to match and strengthen the bag. Add a few pockets to the inside as well that are deep enough for pens, a box of crayons or colored pencils and space for a 12″ ruler. A place for a phone or mp3 player is nice too. Use the fabric to make removable covers for the books. The tote plarn bag is from your recyled bags. The free pattern from myrecycledbags.com . The hardware store sells colored twine that is nice to use. The granny square tote is a nice pick for girls and teens. The pattern is from Hobby Lobby. For boys I would make a flap and use cotton or poly webbing for the straps or to line the straps. Make in a very drab color like grey or taupe. Make it completely plain without the granny squares. Add 1-2 stripes but that is it. Below is a video tuturial to make a messenger bag. Again, the books are heavy. Please line the bags and strengthen the straps with a thin cotton or poly webbing.