Hi,thanks for query.this might be due to excessive salivation.This results from bad oral hygiene.Presence of ulcer due to some vitamin deficiency can also lead this.Please look for the cause and treatment depends on it.In mean time you can use antiseptic mouth wash for rinsing after food.bye.

Hello Anne, Welcome to HealthcareMagic Forum.
It is not at all a serious condition and it does not require any treatment. Saliva is an essential component of the oroesophageal environment and allows for normal speech, taste, mastication, food bolus formation and swallowing. Rhythmic masticatory muscle activity and swallowing are non disruptive events that occur during normal sleep and are reduced hence the saliva which is produced starts drooling outside and wetting the child's pillow. It could happen in adults as well. The smell which emanates is due to the excessive bacterial activity once the saliva is outside and also if the oral hygiene of your child is not good. Make sure she brushes her teeth twice a day, after waking up and before hitting the bed. Schedule regular check ups with her dentist once in 6 months for a minimum. Let her not sleep on her stomach but either to her left or right lateral position with the palm under her cheek.

I Wish her Good Health, Take Care.