Each person on earth has a guardian angel who watches over him and helps him to attain his salvation. Angelical guardianship begins at the moment of birth; prior to this, the child is protected by the mother's guardian angel. This protection continues throughout our whole life and ceases only when our probation on earth ends, namely, at the moment of death. Our guardian angel accompanies our soul to purgatory or heaven, and becomes our coheir in the heavenly kingdom.
Angels are servants and messengers from God. "Angel" in Greek means messenger. In unseen ways the angels help us on our earthly pilgrimage by assisting us in work and study, helping us in temptation and protecting us from physical danger.

Be alert in your every action as one should be who is accompanied by angels in all your ways, for that mission has been enjoined upon them. In whatever lodging, in whatever nook or corner you may find yourself, cherish a reverence for your guardian angel. In his presence do not dare to do anything you would not do in mine. Or do you doubt his presence because you do not see him? Would it really help if you did hear him, or touch him, or smell him? Remember, there are realities whose existence has not been proven by mere sight.
Brethren, we will love God's angels with a most affectionate love; for they will be our heavenly co-heirs some day, these spirits who now are sent by the Father to be our protectors and our guides. With such bodyguards, what are we to fear? They can neither be subdued nor deceived; nor is there any possibility at all that they should go astray who are to guard us in all our ways. They are trustworthy, they are intelligent, they are strong — why, then, do we tremble? We need only to follow them, remain close to them, and we will dwell in the protection of the Most High God. So as often as you sense the approach of any grave temptation or some crushing sorrow hangs over you, invoke your protector, your leader, your helper in every situation. Call out to him and say: Lord, save us, we are perishing. —St. Bernard
Below is a timeless prayer to one's Guardian Angel, that children can learn, but can continue to pray all our life. A partial indulgence is attached to those who recite this prayer.
Prayer:
Angel of God, my guardian dear, to whom God's love commits me here, ever this day (or night), be at my side, to light and guard, to rule and guide. Amen.