Thank you. I just bought a book about linguistics so I can better understand all these terms. Sometimes the audio in other languages is misleading. I am getting a sense of the letter l in Czech.
One is literally a boy that is small, the other is a diminutive.
Diminutives have their connotations, they are often not neutral. Chlapeček can bee indeed used neutrally for a very young boy (depending on the speaker it could be below 3, or below 5 but for me certainly before school) and it can also be used in a condescending way. I even see a small difference to holčička, for which even Google returns pictures of even school-aged girls. Noticable older to pictures of chlapeček.
And then you have hošík, klučina, klučík, děvčátko, dívenka. All these have to be analysed individually as their actual usage might differ and might do so even between regions or speaker age groups or just individual speakers.
Malý kluk can also be used (and often is) in a condescending way. Malý chlapec less often so as chlapec is quite formal these days.