Both MAN-DXB and DXB-BKK operate 2 class a380s so I am not sure which one you are referring to regarding the choice of sitting upstairs but to put it bluntly if I was being selfish I would choose upstairs if I was thinking about other pax I would sit downstairs. (You can probably tell I don't have kids!).

Upstairs is a lot more intimate, with two relatively small economy sections with business to the rear. If the flight is not full usually the rear economy section is closed off and then once you are in the air pax generally move to this area to spread out. Unless you have status there is an additional cost to sit upstairs unless you are assigned these seats at check in. (no idea about bassinet seats).

Immigration has improved in recent times at BKK but I would rather be at the front of 500 pax disembarking the aircraft rather than at the rear. This is a benefit of sitting in the front section downstairs vs upstairs. Also the toilets at the front of the aircraft downstairs are larger.

Someone else with kids might be able to give you a better answer!