Being a parent of a 8-year-old, I understand your concerns. But, unfortunately, I think we have to book two tickets for two kids. IR does not term it half-ticket, it is one full seat/berth at subsidized fare, thats all. IR does not mind them going waste. Anyways it will not as the concerned TTE will grab those opportunities. Above 5 years, you do not necessarily carry an ID proof for the kids, though if they are booked as separate tickets, yes you have to.