They cleaned there teeth with salt.There parents didn't let there kids.
=> They cleaned their teeth with salt. 

>> Who cleaned there teeth with salt? Who are THEY?
Remember, in making a paragraph you have to provide clear information. If are going to use preposition in sentence make sure that you already mention the subject in your first sentence. This could lead confusion to readers.

They cleaned there teeth with salt.
=: In the old days, adults cleaned their teeth with salt.

CLAIRE: There parents didn't let there kids. WHAT???
=: They didn't let their kids do it.
('They' represents 'adults' )

There and their are two different things. There is used of you are saying something exist.
 EX. There is a ball on the table.
       There are footprints on the sand.

Their is used if you are saying that something belongs to someone.
EX. Their dog, Bruno, is sleeping on the sofa. 
      Mr. and Mrs. Samson are cleaning their house.