I was a kid but not too small was not mature but at age of 12-15. when my father came to school I said to my friends that he is the driver of mine. what is the punishment of it? Will Allah forgive.
Allah give the parents a very high status in the Holy Quran, and linking his worship with the obedience and good treatment for parents
And your Lord has decreed that you not worship except Him, and to parents, good treatment. Whether one or both of them reach old age [while] with you, say not to them [so much as], "uff," and do not repel them but speak to them a noble word.
وقَضَىٰ رَبُّكَ أَلَّا تَعْبُدُوا إِلَّا إِيَّاهُ وَبِالْوَالِدَيْنِ إِحْسَانًا ۚ إِمَّا يَبْلُغَنَّ عِندَكَ الْكِبَرَ أَحَدُهُمَا أَوْ كِلَاهُمَا فَلَا تَقُل لَّهُمَا أُفٍّ وَلَا تَنْهَرْهُمَا وَقُل لَّهُمَا قَوْلًا كَرِيمًا
and If there were less than the word "uff", It certainly Allah mentioned this word in the Holy Quran.
Such this word, which does not exceed three letters, Allah forbade sons to say this word to parents.
How about your said to your friends that he is the driver of you?
however, if you do not know this when you were a kid, then it is okay Insha'Allah.
And Allah knows best!
I think it's good to do something that your father's loves. and in your intention is to Allah forgive you,
without telling your father anything.