Children typically begin chewing on furniture as a result of teething. A safe preventative measure is to purchase a teething guard crib rail.  Providing children with alternatives is also beneficial.
 
I recommend making a tasty & natural teether following these easy steps:

• Boiling chopped apples (with a few sprinkles of Ceylon cinnamon assuming no allergies)

• Place spoonful of it in a thin baby washcloth and tie it off with a hair rubber band

• Put it in the freezer for 2 hours

 
To help ease the teething pains, you can mix ground cloves with coconut oil and rub on their gums as a natural numbing agent. 
 
As far as the behavior, it is best to tell your children, “we don’t chew [furniture item], but you can use this [alternative teether].” Repetition is key with children in terms of understanding what they are and are not allowed to do. 
 
In terms of treating furniture, you want to make sure that the painted furniture items are made with non-VOC paint (volatile organic compounds). VOC paints have been correlated with respiratory illnesses and have a toxic odor, irritating the eyes, nose, and throat. Paints are currently not made with lead. However, if you are using hand me downs it is best to ensure that they are lead-free, as consuming paint with lead can lead to neurological conditions. 
 
While more rare, if you observe your child ingesting inedible materials (such as clay and dirt), this could be a condition called Pica. You should inform your medical provider if this is happening.