I have had parents who asked me about their kids i.e. their eldest, the middle child and the youngest lad. What I have found is that the 5th generally represents the eldest child. Interestingly, I encountered Chiron on the cusp of the 7th or turned 3rd. The eldest child used to have a stammer when he was young.
According to John Fawley’s Horary Textbook, the subsequent child(ren) are the turned 3rd of the 5th. The reason being any subsequent child is the sibling of the 1st child which is represented by the 5th.
Usually I would give a physical description to make sure that I’m “reading” the correct child properly.