Is $20 target or Amex gift card is a good Christmas gift for preschool teacher?

No, that's a perfect gift. I'm a teacher and I love homemade gifts too. I do get a lot of the same thing over and over, so a nice gift card is great because they can buy whatever they need (instead of 30 lbs of chocolate that I will inevitably eat and blow my diet.) Not to complain though, it's the idea that counts that the parents appreciate the efforts you put forth.