Costumes will be the most important part of your skit. If your characters are not well dressed or made to look like the thing they are pretending to be, you will lose your audience. Humans are very visually motivated. If your characters are pretending to be average people going into a dentist's office, costume ideas can be taken from current magazines and television shows. If the characters will be things like teeth, floss or other dental hygiene items, it may be necessary to dress them up in mascot costumes.
Your story must be planned before your dialogue. If you do not know how the characters are going to get from the beginning to the end, you will not know what they should say or do on the stage. Write the outline of the story on a piece of paper before you attempt to write your script. Stories range from a tooth being attacked by a cavity monster to more adult circumstances like actors portraying a root canal and the recovery afterward.
Deciding on your target audience before writing your script and making your costumes. A young child will be better informed if they are also entertained while they are learning, but few adults would want to watch a children's skit with dancing teeth singing a song about flossing. If there is a way to combine two extremes such as dancing, singing teeth and an adult message about how cavities are drilled and filled in, this will give you a skit that will speak to all of your target audiences at once.
The venue will matter greatly on what sort of production value your skit will be allowed. If you are given a bare stage in the middle of an auditorium packed with children, you may only be able to give the basics of your skit without any attendant music, audio or lighting. If you are filming the skit for future use, you will have the ability to stop and refilm parts that do not turn out well.