Jeremiah 1:6–7 KJV Then said I, Ah, God! behold, I cannot speak: for I am a child. But the Lord said unto me, Say not, I am a child: for thou shalt go to all that I shall send thee, and whatsoever I command thee thou shalt speak.
In Jeremiah 1:4–5 God makes a call to Jeremiah but Jeremiah exclaims that he is a child. For that reason God tells him not to call himself a child.
Jeremiah underestimated himself, and He did not see how God saw him, nor did he recognise who was calling him. For that reason God remarks to him «Say not, I am a child» because God did not see it like that, God saw him as a male, and then God commends that he be subject to everything he orders, that is, God marks his preeminence in this.
But if we look at what the word «child» means, we see that if comes from the word נַעַר náar H5288 which has several meanings: servant, boy, young, lad, child. With this we see that he not only underestimated himself for his age (youthfulness), but also because of its immaturity (child), social status (servant) and inexperience (boy).
But God did not consider that, God only considered the following:
Jeremiah 1:5 KJV Before I formed thee in the belly I knew thee; and before thou camest forth out of the womb I sanctified thee, and I ordained thee a prophet unto the nations.
God did not care that Jeremiah had a concept of himself as a slave, immature or inexperienced, to God the only thing that mattered was the knowledge He had about Jeremiah.
What do they mean, the words of God «Say not, I am a child»?, means: «I know what I have done in you, I know what I have formed in you, I know that I put my spirit in you, and I know you’re my prophet, so do not tell yourself child, because you have the ability to go where I send you and you have the capacity to say what I say you will say, because I know what I have done in you, formed in you and placed in you.»
[H5288] נַעַר, náar, from H5287; (concretely) a boy (as active), from the age of infancy to adolescence; by implication a servant; also (by interchange of sex), a girl (of similar latitude in age): babe, boy, child, damsel [from the margin], lad, servant, young (man).