I never understand why people get so paraniod over personalizing children's things.
Kids that little are rarely, if ever, alone(like you, we have to drop off and pick up --all the way through 1st grade actually).
And if they do somehow get to be alone away form you at some time, what are the odds that they will have their backpack on at the time, and be in the presence of someone who wants to use that information to try and get their attention so they can kidnap them.
And when they are with us, we say their name aloud, so people could find it out a lot easier than peering on their backpack.
Regardless, every preschool class my kids have been in, we were asked to write their names on their backpacks, so you can either monogram it, or end up writing it on with a Sharpie. LOL
I say do it.