Here is some info:
Praise be to Allaah.
With regard to the prohibition on sleeping on one's stomach, it was narrated by Ahmad and Abu Dawood (5040) that Takhfah ibn Qays al-Ghifaari said: When I was lying on my stomach in the mosque just before dawn, a man nudged me with his foot and said: “This manner of lying is hated by Allaah.” I looked and saw that it was the Messenger of Allaah (peace and blessings of Allaah be upon him).
Ibn Majaah (3724) narrated from Ibn Takhfah al-Ghifaari that Abu Dharr said: The Prophet (peace and blessings of Allaah be upon him) passed by me and I was lying on my stomach. He kicked me with his foot and said: “O Junaydib! This is how the people of Hell lie.”
The ahaadeeth which speak of the prohibition on sleeping on one’s stomach vary in their degree of soundness. Some scholars classed them as da’eef (weak) such as al-Bukhaari, al-Daaraqutni, Ibn Abi Haatim and Ibn Rajab, and others classed them as saheeh or hasan, such as Ahmad Shaakir in Tahqeeq al-Musnad (14/248-249) and al-Albaani in Saheeh Abi Dawood.
Sleeping on one's stomach is harmful from a health point of view, for both young and old. There are medical reports which speak of a high death rate among infants who suffocated because they were put to sleep on their stomachs, as well as allergies because of the nose being too close to the bedding, if the bedding is made of sponge or wool and collects a large number of germs and microbes.
Yes, sleeping on the stomach is good for expelling wind, but always doing that is harmful, as stated above.
The child should become accustomed to sleeping on his right side, because this is the best manner of sleeping, and is in accordance with the Sunnah of the Prophet (peace and blessings of Allaah be upon him), and is beneficial for the body.
It says in a medical report: Sleeping on the right side is the healthy position, because the left lung is smaller than the right, so the heart is under less pressure and the liver is settled and not suspended, and the stomach is resting comfortably on top of it, which as we have seen makes it easier to eliminate the food that is in it after it has been digested. Sleeping on the right side is also regarded as the best medical procedure that makes it easy for the tubes of the left lung to expel mucus discharges.
As for sleeping on the left side, it is not acceptable, because in that case the heart will be beneath the pressure of the right lung, which is larger than the left, and this will affect its function and make it less active, specially in those who are elderly. The full stomach will also be under pressure, which will increase the pressure on the heart and liver – which is the heaviest organ – which will not be fixed, rather it will be hanging, as it is located on the right side, so it will put pressure on the heart and on the digestive tract which will delay elimination.
Experiments have proven that the passage of food from the stomach to the intestines takes between 2.5 and 4.5 hours, if a person is sleeping on his right side, but it takes 5 to 8 hours if he sleeps on his left side.
See: al-Tibb al-Nabawi fi Daw’ al-‘Ilm al-Hadeeth by Dr. Ghiyaath al-Ahmad, and al-Nawm ‘ala al-Jihah al-Yumna by Dr. Ibraaheem al-Raawi.
So children should be trained to sleep on their right sides, following the example of the Prophet (peace and blessings of Allaah be upon him) and so as to benefit their bodies.
The scholars of the Standing Committee were asked: is it true that it is bad to sleep on the stomach? I was told that sleeping on the stomach is makrooh because that is how the shaytaan sleeps, but I have been used to sleeping in this position since I was small, and I still do that now because it is my habit. What is the ruling on that?
They replied: It is makrooh to sleep on the stomach, because of the report narrated by Abu Dawood from Takhfah ibn Qays al-Ghifaari, who said: When I was lying on my stomach in the mosque just before dawn, a man nudged me with his foot and said: “This manner of lying is hated by Allaah.” I looked and saw that it was the Messenger of Allaah (peace and blessings of Allaah be upon him).
Narrated by Abu Dawood in a chapter entitled: “A man who lies on his stomach.” So that should be given up even if it is one’s habit, because it is prescribed for the Muslim to give up habits that go against sharee’ah. End quote.
Fataawa al-Lajnah al-Daa’imah (26/146).
And Allaah knows best.