We flew in Feb with our then 4 year old and 2 year old twins. Was really worried about the flight but it went really well. People around us didnt even now we had 4 kids with us until we got up to leave the plane. I would take a spare change of clothes as well for your eldest just in case, ours was sick on second journey and I had a spare pair of shorts but no pants, or shoes which she managed to cover. I personally wouldnt take to much stuff on long plane journey again, we took little bags full of bits and piece to keep them occupied and we never even opened them. Singapore airlines give the kids plenty of things to keep them occupied. It just another thing to carry on and off the planes.
Good luck on your trip and hope you like what you see.