When the Kid was in the baby room at his old preschool he practically shared a wardrobe with one of the other little boys in his class. The other child's mom and I casually developed an agreement just to do each other's laundry and not worry about it.
Her: Have you seen Jimmy's Snoopy shirt? We seem to be missing it.
Me: Yup--I'm so sorry, we're just a little behind this week. I'll get it washed for tomorrow, I promise!
I think it's just a fact of life when your kids are that age. The clothes get mixed up. And I don't blame the teachers a mite for not being able to keep it all straight. I don't know what clothes belong to the Kid some days. If I had 10 kids to remember? No way.
We do lots of permanent marker and labeling to try and help, but--when I have a little extra time--I've found that one of these little monograms is a surefire way to get the right clothes back.
To make one, using contrasting fabric, cut out the first letter of your child's name. Then, pin it to your child's pants--the back pocket is a good spot, as is the bottom hem--and stitch it on using embroidery floss.
If you're handy you can machine sew it, but I think it's about as fast to do it by hand. Plus I like the slightly wonky look, which only gets better with a few spins through the washing machine.
Also, it's darned cute. And anytime you can do "cute" and "practical" at the same time, I believe you should jump all over that.