If I killed a child - when I was a child will God forgive me if I repent now ?

That question omits several pieces of information that would be needed to provide a complete answer. Depending on age, circumstances and other factors, it may not even have been a sin. In any case, God forgives every sin that we truly repent of.

God forgives all sin, There is no sin God won't forgive, look at Paul he was a "murderer and a blasphemous man". God and Jesus forgave him, and he converted more people into believers than any of the 12 deciples.

Yes...and it has to be genuine...

A loving god can replace that life...and will in the resurrection...

(Revelation 20:12, 13) And I saw the dead, the great and the small, standing before the throne, and scrolls were opened. But another scroll was opened; it is the scroll of life. The dead were judged out of those things written in the scrolls according to their deeds. 13 And the sea gave up the dead in it, and death and the Grave gave up the dead in them, and they were judged individually according to their deeds.

God always forgives.

All your sins have been forgiven on the cross. That's not the question. It's whether or not you choose the intimacy and responsibility of that relationship.

A better question, could you forgive yourself?

Yes, if it is true repentance.

All gods are myths.

Yup.