Why is "jste malí chlapci" wrong?
It isn't, it is accepted. Please use the "my answer should be accepted" report feature.
It says that the correct spelling is "jste malí chlapečci." Is that right?
Well, chlapeček is the diminutive of chlapec, so it is a little boy by itself. But malí chlapečci is also fine for little boys. It likely offered you the closest thing it had since you likely had some typo in "malí chlapci".