According to many child psychologists children begin to experience nightmares between the ages of 3 and 6. It is very common for children to experience this, and in fact is a part of their growth. Usually at this particular stage the child is starting to become more aware of the world around them, and this is sort of a reaction to this curiosity, and questioning of what the world is about.
 The challenge for the parent, however, is to be able to get the child through this stage and not have it be a year after year difficulty. One way to cope with this challenge is to talk to your child about it. Try to go over the dream, and recreate it with them, in a way that has the story line end on a positive note instead of the frightening way it may have ended in the dream. For example, if your child was afraid of a tall stranger in the dream, have the stranger end the story by sitting down with the family and having a friendly cup of tea.
 Another aspect of this issue is that of having the child stay in mom and dad's bed after they've had a bad dream. In this instance experts say that it is best not to encourage this too much. In fact, by having the child sleep in your room you are saying that there is something to be afraid of in their room. Again, try to get the child used to sleeping in their own room, and over time they should be able to outgrow this stage.
 Finally, if your child is not having run of the mill disturbances in their sleep, for example if it's sleep walking  or sleep apnea, consult with a pediatrition. Some of these health issues can run in the family, and with proper attention can be taken care of. Again, sleep disturbances are not uncommon for children between the ages of 3 and 6, and they can be resolved.