While it's most common in stage and screen acting to have children's roles portrayed by child actors, there are some times when adult performers are asked to take on the task of pretending to be children. Roles that give a caricature portrayal of a child or works of art containing subject matter too inappropriate to expose a young person to require a performance from an adult actor. This task is generally considered to have a high margin for error, so those facing the challenge should take the time to carefully study for this special kind of role.
Determine whether the role you're playing is meant to come off as realistic or a caricature. This will depend more upon the style of play and theater than whether or not the subject matter or character of the show is serious or funny.
Choose and study a particular age group. Children of different age groups will have different habits, mannerisms, and levels of social skill and ease, so it's important to have a specific age group in mind. You can supplement your study of actual children with some reading about child development at various ages.
Increase your energy level. Young, growing bodies of children move easily and burn a lot of calories, in addition to being smaller and less weighed down by gravity. Reflect this difference in your movements by moving slightly more quickly and more often. Work on developing a higher level of physical relaxation in your muscles.
Fidget. Children fidget more than adults because of higher energy levels, but also because they haven't been fully socialized not to do so. Be sure to choose fidget gestures that are more childlike in quality, such as pulling on clothes, scratching itches, chewing lips, or bouncing and shuffling feet. This is as opposed to "adult fidget," which tends to include more affected gestures, such as adjusting glasses, straightening hair or toying with pens.
Train your physical movements to be less inhibited. While adults in social situations will generally stick to a more narrow and prescribed range of movements, children will have far fewer qualms about activities such as lying on the floor, rolling around and sitting in odd positions, or physical activities such as running, jumping or standing on one leg. Children also are more likely to use movement in ways that are immediately convenient rather than politely conventional, such as leaning and reaching over a dinner table or tipping their heads upside-down over the back of a chair to talk to someone behind them rather than turning around.
Speak in the lighter range of your voice. Don't use an affected falsetto (unless you're creating a caricaturish interpretation), but stick to the lighter side of your natural speaking voice. This will give an impression of youth.
Affect a more childlike inflection to your voice. Children's speaking inflection, especially that of very young children, is not as sophisticated as that of adults. Children tend to speak in voices that tend toward monotone during times of low emotion and contain greater, more uncontrolled extremes of pitch during times of high emotion and excitement.
Interact appropriately with "adult" cast members. Though individual characters may vary, children in general will be more deferential in voice, face and body language when interacting directly with adults. When not interacting directly, children will tend to pay less attention to adults than adults do to one another, and stay more focused on other things such as playing with toys, daydreaming or observing objects in their vicinity.