Have you ever noticed that movie characters never have one or two year olds? They have babies all the time. They certainly have teenagers and school age kids. Sometimes they even have three or four year olds, but they never have toddlers. This is because toddlers are uncontrollable balls of destruction that you cannot communicate with, and no director wants that on their movie set.
Babies are easy. Once they are born, they basically just lie around for 6-12 months. Oh, sure they scream and flop about, but there are plenty of times when they sleep, or sit in your arms and stare at the world. This is perfect for movies, because all movies require of babies is that they look cute and don't move much. Sometimes they do ask a 3 or 4 month old to cover itself in goo and pretend that it was just born, but this also works well, since the baby is supposed to be crying at this time, so it is not a problem to upset the infant.
Kids and teenagers are also generally not a problem. They speak english and will, at times, do what grownups say. The younger they get, the harder it is to find decent actors, but often you only need the little kids to run around screaming in the background (which is what they would be doing anyway), and if you look hard enough you can find a Dakota Fanning or a Haley Joel Osment. They are out there.
What you cannot find, no matter how hard you look, is a one or two year old that is interested in doing anything you say. You can't even depend on them to run around screaming reliably. The most futile thing a screenwriter could ever do would be to write a movie scene in which a toddler is supposed to do something. If you want the toddler to walk from one room to another, the toddler will probably sit on the floor and giggle. If you want the toddler to sit on the floor and giggle, the toddler will probably pull the hair of Megan Fox. If you want the toddler to yank on Megan Fox's hair, the toddler will probably bite Shia Lebeouf. You get the picture. The only movie in which I can conceivably picture a toddler working well, would be a mockumentary in the vein of "Best in Show," in which the toddler is free to improvise its own parts.
So next time you watch your favorite movie, take note. You will not see any toddlers. It's not worth the hassle. And also, people with toddlers do not do anything that would make an interesting movie. Unless you want to watch a movie about smearing food on things and changing the settings on the DVD player.