You get massive respect from too me for fostering

We travelled to WDW in 2007 when my youngest daughter was 19months. We did buy a seat for her on the flight and this was the best thing we did

I know it's an extra expense but I really thought it was worth it. If you're flying with Virgin you can get a special seat that straps on the seat to "boost" your toddler, we used this going out (you request it once onboard) but didn't bother coming home.
I took her favourite bear that she sleeps with, a blanket, loads of toys (her sister was just 4 at the time) and colouring books for both of them, healthy snacks as well as naughty ones and sugar free lollipops to suck on take off and landing (helps the ears to "pop"). We took them for regular walks up and and down the plane but with DH and I taking the outer of the four seats they were happy to have the armrest up between them and have their own little play space. They also watched the childrens TV and films which passed a couple of the hours.
We were very lucky our girls were very behaved despite only each sleeping for about an hour. It really wasn't that bed and the time went quickly.
Another tip that worked for us, if the aircraft you're flying on has the small economy cabin between premium and the main economy cabin, try to get seats in there. The main cabin get noisy and loud and my girls are easily distracted by children that, lets say, aren't being so well behaved
Good luck, enjoy your trip.
I'm sure I've missed loads out but if you want to know more, just PM me.