"Is fantasizing about having sex with child a sin that will keep me from recieving my great reward???"

There is little doubt in my mind that the answer to this question is "NO".

God gave His son Jesus for ALL your sin. He has separated you from the law of sin and death.
The law of sin and death was: if you sin you die.
The law of grace is: if anybody does sin, we have one who speaks to the Father in our defense—Jesus Christ, the Righteous One. (1 John 2:1).

You still sin, you still deserve punishment, the difference now is that Jesus has you covered. That is why there is now no condemnation to those who are in Christ Jesus.

The "who do not walk according to the flesh, but according to the Spirit" is not conditional.
It is not "as long as you walk in the Spirit"
rather it is "if you HAVE the Spirit" then you are "not in the flesh" (Rom 8:9)

This gives us freedom from sin's consequences to a large degree before God. That is... you can't lose your salvation over it.
Holiness is another issue.
We cannot use this freedom as an excuse to indluge the flesh. "For you, brethren, have been called to liberty; only do not use liberty as an opportunity for the flesh, but through love serve one another." (Gal 5:13)

So we strive (in Godly fear) to overcome sin that we may honour the Father who took the consequences of our sin and laid it on His own Son's back. Your salvation is secure in Christ, the Holy Spirit is your guarentee... just beleive.

Blessings
Cat.