God, the author of the biggest holocaust

[Randy Carson]

The 'holocaust of the children', as Greg Paul puts it, dwarfs any human caused holocaust by a wide margin. All of the mass slaughters of the 20th century combined pale by comparison. 

While I might agree that this is one of the tougher questions proposed by atheists, there are coherent responses.

I'll give you the condensed version, but I recommend you read the full version here:

Hard Sayings Of The Old Testament
by Jimmy Akin
http://jimmyakin.com/2007/02/hard_sayings_of.html

Akin explores this subject in some depth, so grab a cup of coffee and block out some time to get through it all.

Short version:

God has granted to each person an infinite life: x amount of years on earth and infinity hereafter. A person who lives to 100 has the same amount of life as the child who is slaughtered at the age of two: infinite years. Their lives would look like this:

Man dying at 100: 100 + infinity = infinity
Child murdered at two: 2 + infinity = infinity

Now, since God is more than capable of making sure that the child who dies at two has an infinite amount of joy and happiness of an infinite number of years, that child has not been cheated out of anything by not living to be 100. If anything, that child may have been spared 98 years of suffering and pain.

Consequently, God is equally fair and generous to all of His beloved children regardless of their circumstances in this life.

Akin provides a lot more than this, of course.

Hope this helps.