Why Does My Child Start Foaming In The Mouth While At Sleep?

Why would a child, age 5, have foam in their mouth sometimes while sleeping in the day time(past three days) only.. 1.Is it foam or saliva?- it is foam 2.Does he have good oral hygiene?- yes 3.Does he take some sweet food items before going to sleep?-no 4.Does he have any other problem other than this?-no