Part I of the family vocab series is here.
“Child” (also “baby”) is طِفل (ṭifl, plural أطفال aṭfāl) if a boy or طِفلة (ṭiflah) if a girl (plural طِفلات ṭiflāt but ONLY if they are all girls, otherwise use the masculine plural).
“Son” is ابن (ibn, plural أبناء abnāʾ).
“Daughter” is بِنت (bint, plural بَنات banāt), which can also mean “girl,” though there are other words for “girl” as well. You may also encounter ابنة (ibnah, plural ابنات ibnāt), though this is more formal and less common.