Hi, we travelled several times with a young child of similar age about 2 years ago; similar length of time.
My plan of attack was: 1. try and work to a routine even though travelling; so when lights go out encourage sleep - it can take a while but just follow normal sleep routine. We did bottle, books, and a dummy and made sure the favourite toy was always there.
2. lots of small snacks, anything that keeps well like dried fruit. We used these for take off and landing (really important for the ears); either eating or drinking at these times for the child.
3. some travellers love children, this may mean a person who will just play hide and peek over a seat for a while. If a person looks inclined just leave them to entertain your child for a while.
4. Ask for a bassinet seat (up the front of sections); you get lots more leg room even if you decide not to use the cot. And ask if there are spare seats on the flight, and whether you can have an extra one. It sounds cheeky, but we found check in staff were only too willing to block off a seat in a row of 4 if there are 3 of you travelling (for example); no charge.
Try not to worry, it always went better than we anticipated, but a bit tiring.