Camera
If your child disappears, are you going to be able to describe to the police how your child looks like properly? I don't know about you but I am lousy at descriptions. Also think about how long it would take for you to describe your child.
Majority of people have phones with a camera these days. When you dress your child to take your child out of the house, take your phone and snap a picture of your child.
If your child turns up missing, you can show the picture to the police officer and he or she will immediately know what your child looks like and the clothes your child is wearing.
If your child doesn't turn up missing (which I hope is always the case!) then you will have a photo to add to your collection of memorabilia!