Babies will at some point learn to roll onto their front. When the baby can roll from back to front and back again, on their own, then leave them to find their own position. At the start of any sleep time, put them on their back. Yes, and not only is it safe for your baby to play on their front but it is to be encouraged so that their muscles develop properly. Despite their widespread use there is no research evidence that monitors, also known as apnoea or breathing monitors, prevent cot death.
Babies can and do die whilst on a monitor.
They are designed to sound an alarm after 20 seconds if they can't detect a baby's breathing movement. They may use sensor pads attached to the tummy or housed in an elastic belt which fits over the baby's clothing. Others use a pressure pad under the baby or an ultrasound beam. They do not monitor air flow and therefore can not detect an obstruction in the airway until breathing movements cease. In the past, there had been concerns that bumpers may make babies too hot, increasing the risk of cot death.