How do Trinitarians explain the Father granting life to the Son (John 5:26)? For as the Father has life in himself, so he has granted the Son also to have life in himself. (John 5:26, NIV)


How do trinitarians explain the fact that the Father has granted life to the Son?
The "begetting" here is understood in an eternal sense, and is not a matter of being made or created. The creeds phrase it as eternally begetting (fathering), or pretemporal begetting, or an eternal proceeding from. And they are explicit in that it is not a matter of being made or created.

From the Athanasian Creed, 


  The Son is of the Father alone; not made, nor created, but begotten. 


And,


  So there is One Father, not Three Fathers; one Son, not Three Sons; One Holy Ghost, not Three Holy Ghosts. And in this Trinity none is afore or after Other, None is greater or less than Another, but the whole Three Persons are Co-eternal together, and Co-equal. 


And from the Nicene Creed,


  And in one Lord Jesus Christ, the only begotten Son of God, and born of the Father before all ages. (God of God) light of light, true God of true God. Begotten not made, consubstantial to the Father, by whom all things were made.


The Son proceeds from The Father and is therefore of the same eternal substance. This differs from created entities, which are different in "substance" from their creator. The Father is the eternal cause of the Son; but the Son is not created or a distinct substance from the Father. The Father and the Son, with the Holy Spirit, are one in being.