Advance train ticket booking for a family of four (one kid under 1.5m)

I need to book four tickets (soft sleeper) for my family for Jul 17 departures from Beijing to Yongan. 1) When should I make the booking? 2) I will be arriving Beijing on Jul 10. If I book as soon as I arrive Beijing, will the tickets sill be available? 3) One of my kid is 8 years old, under 1.5m. If I book online here, how do I indicate he is entitled for a child ticker (for a soft sleeper). Thank you.