I've come to the conclusion that babies and toddlers do not obey the laws of physics. How can a 20 month old that was right next to you two seconds ago suddenly be half way across wal-mart? The answer is not one parents want to hear. The answer is wave functions and quantum tunneling.
See, scientists will tell you that little things like electrons don't have a specific place where they are. They have a probability field. Once someone actually looks to see where an electron is, the probability field collapses, and it picks a spot. Electrons also have another weird quirk. They can be either a particle or a wave depending on what a scientist is looking for. Little things also have an ability to jump across a place where they are to somewhere else without going through the space in between.
I've decided that small children and babies are too young to collapse their own wave function. An adult observer is required. Thus, small children can disappear as soon as no one is watching and turn into a wave of pure energy. Once an adult starts looking, they pick a location and often end up an impossible distance away, like across a store or on top of a refrigerator.