It is impossible for a child to smother himself by lying face down on a bed with no pillow or getting under the bedclothes. He will come up for air when he needs to. What you need to be worried about are thin plastics since they are both airtight and clingy. If your child puts a plastic bag or a thin plastic sheet on his face, the first breath he takes in will make the plastic cling to his face in such a way that he will find it difficult to remove it.
In such a situation, you have to act fast. Rip off the plastic and all will be well. He may gasp for air but he will be fine soon.
If he is not breathing, do not waste any time. Start artificial respiration if you know how to, and give him six full breaths and call for an ambulance. If you do not know how to do it, do not try it, call for help.
If it is earth, sand or cement that has got to him, efforts to make him breathe will only [push the foreign material deep into his air passages. Call for the doctor immediately.
Even a new born will turn his head if he cannot breathe. But this may not help. Remember don't have any plastic around the baby. Treat them like poison and avoid using them.
Never let him tunnel in sand or piles of earth. Do not encourage games of burying each other in the sand at the beach. Sand dunes are great to look at, but they can be very dangerous too.
Be careful of everything your child can get into but cannot get out of. Keep washing machines and refrigerators locked. You will not even hear him call because of the insulation. Do not encourage him to play hide and seek inside cupboards.