I fly once or twice a year with my 2 kids starting from 18mo and 3.5 and now they are 3 and 5 (usually on my own). So will tell you what usually happens with us. We usually have a morning flight leaving the UK about 10 ish so wake them up and get to the airport as a normal morning routine. I let them do what ever keeps them happy and quiet on the plane. After the meal service etc I try to get the younger one to settle down and take a nap and have been lucky enough that the older one will usually take one too at some point. So far in my experience they will sleep for 2 hours or so, I have taken along an antihistamine to give them just to calm them down a bit but it doesn't make them sleep longer just relaxes them some to dose off.
We fly to Chicago so 6 hours different. Once we land they are usually ok with all the excitement of seeing everyone, toys they haven't seen in 6 months etc to not be too sleepy even though their body clocks are close to bedtime. I usually just let them do whatever they want until they start acting tired then do their bedtime stuff. I can usually keep them up till 6 or so which isn't too bad.
They next morning is usually a crap shoot as far as when they will wake up. I've had them get up wired at 3 am then take a nap at 9am. had them sleep as late as 5am and be back on track for a normal day. Usually after that I can get their day adjusted and to bed at a normal time the next night which gets us closer to a normal wake up time. It's never take more than that extra day in my experience. I've pretty much had the same when we fly back to the UK. When they do wake up at odd hours I try to keep things calm and quiet too.
I wouldn't be afraid of what will happen just go with the flow and adjust as you need to. Don't be afraid to keep them up a little later or wake them up from a nap if you think it will mess up the rest of the day. Also after they go to bed I am ready to sleep too (I used to be able to nap on the plane some but now with the kids that doesn't happen). This way you are ready if they do wake up early and also you are adjusting too. Also my mom is usually an early riser so will keep them busy if I need to nap a little more.
hope that helps a little