Please tell me how can I explain Satan to my three-year-old child?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

Satan is a creature amongst the many creatures of Allah. We can not see him in his original form (as a Jinn) but he is capable of taking many forms that can be seen. Allah Says (what means): {Indeed, he sees you, he and his tribe, from where you do not see them.} [Quran 7:27]

There are many creatures that we cannot see but we can know of their existence. The soul can be felt which is in the body of every living being.

There are many other things we cannot see though they undoubtedly exist such as intellect and electricity; we can not see them, but we can observe their effects. So, Satan also exists, and he is the fiercest enemy of human beings. Allah Says (what means): {Indeed, Satan is an enemy to you; so, take him as an enemy.} [Quran 35:6]

This enmity emerged at the moment when Adam, may Allah exalt his mention, (the father of all human beings), was created and Allah Commanded the Angels to prostrate to Adam. Allah Says (what means): {And [mention] when We said to the angles, "Prostrate to Adam," and they prostrated, except for Iblees. He said, "Should I prostrate to one You created from clay?" [Iblees] said, "Do You see this one whom You have honored above me? If You delay me until the Day of Resurrection, I will surely destroy his descendants, except for a few." [Allah] said, "Go, for whoever of them follows you, indeed Hell will be the recompense of you - an ample recompense.} [Quran 17:61-63]

The only thing Satan wants from mankind is to make them join him in his party as mentioned in the Quran (what means): {He only invites his party to be among the companions of the Blaze.} [Quran 35:6]

He always calls the people to evil and wickedness. Allah Says (what means): {He only orders you to evil and immorality and to say about Allah what you do not know.} [Quran 2:169]

But whoever follows the Sharee’ah and places himself in the protection of Allah, Satan can never harm such a person. Allah Says (what means): {Indeed, there is for him no authority over those who have believed and rely upon their Lord. His authority is only over those who take him as an ally and those who through him associate others with Allah.} [Quran 16:99-100]

Only firm belief in Allah helps one escape from Satan's fraud, and receive the Pleasure of Allah and gain Paradise. Allah Says (what means): {And whoever believes in his Lord will not fear deprivation or burden.} [Quran 72:13]

Allah knows best.