Why does my 3 and a half year old drool started drooling about 2 months ago mostly when asleep

The : The most common reason folks (especially around your child's age) drool in their sleep is because they are mouth-breathing. There are many reasons why she/he may not be able to breathe through her/his nose such as allergies, colds, sinus infections, enlarged adenoids and/or enlarged tonsils. Certain medications can also cause increased drooling at night. You should make an appointment with her/his doctor to discuss this problem and have her/his examined. Once the underlying problem is taken care of, usually the excessive drooling improves. Good luck!