Khushi,

Grace unto you, and peace,
from God our Father and the Lord Jesus Christ.

Thank you for the compliment; But I would have you know being a Child is a kNOW not: what not to be. Of child/man, then/now, it is now the age of man kind.
- God said: let us make MAN (not child): Gen 1:26.
- Better blessed the MAN of child/man: Ps 32:1,2
- CHILDren of the kingdom get cast out: Mt 8:12
- Every MAN of child/man presseth into KofG: Lk 16:16
- Christ is head of every MAN (not child): 1Cor 11:3
- Jesus: only a MAN can be born again: John 3
- David: what is MAN, blessedess of MAN: Rom 4:7,8
- When I became a MAN, I put away CHILDish: 1Cor 13
- Higher exhortation is be no more CHILDren: Eph 4:14
So it is no longer a compliment to be Child of God. as noted by Paul calls carnal Corinthians childish.
JC also called hypocrites the CHILD of hell, and
their proselyte more the CHILD of hell in Mt 23.

When did this 'change' from Child to Man occur,
as if the change from servant(prophet) to friend,
and as if the change from law(child) to grace(man)?

According to JC it occured about 2000 yrs ago. Eg:
The law and the prophets were until John (Baptist),
but since that time the kingdom of God is preached,
and every MAN (not child) presseth into it: Lk 16:16.

Eg: note the change in John 15:15
Henceforth I call you not servants... but friends.
(see Gal 4 re child differeth nothing from servant)
(neither child nor servant nor their lord inherits)

In the same manner "through" Jesus-->Christ is
allegorically as if through CHILDren-->MANkind.
Jesus was the head of CHILDren (age of law)
Christ is the head of every MAN (age of grace)

So child of child/man is kNOW not. Be a man.
Servant of servant/friend is kNOW not. Be a friend.
Allegorically law of law/grace is kNOW not. Be grace.
Jesus of Jesus/Christ is kNOW not. Be a Christ-ian.
He taketh away the 1st that he may establish the 2nd.

Peace with God is not even plausible in lamby Jesus.
God is not the author of confusion, but of peace.
Jesus brought division (confusion), not peace.

The grace of our Lord Jesus Christ with you all. Amen.