Hmmm... I think the passengers or the flight attendants would allow you to swap seats so that you can sit together with you child, even if your seats are separated. No people would want the child to be sad/cry during flight.

1. You pre-purchase your seat when you buy your ticket.
2. Web-check in 14 days before your flight when most of the people still haven't check in yet. You can see your assigned seats during the web-check in, and you still could purchase your prefered seats that time. Most of the seat that time are unassigned so you would have plenty of choice.
3. Ask around after boarding to swap seats. Most of the time my seat are 1-2 rows apart from my wife or friends if they assign different seats for us (more than half of the time AirAsia just put us side by side.)