We recently went on a four hour flight with our 17 month old daughter, and although she really was an angel, it was still a stressful experience for me as a pregnant mother!
So here are my suggestions to make the experience less stressful.
First of all when you are packing your hand luggage, make sure you have more than enough nappies, wipes, a travel change mat (it's easier to change your baby's nappy on your knee rather than in the tiny aeroplane toilet), a couple of changes of clothes, for you and baby, and any other essential items that you carry for your baby. Some reseal able plastic bags will also come in handy for any soiled items
Ensure you have plenty of snacks for your little one; I took grapes, raisins, breadsticks, crisps, jaffa cakes, crackers and some chocolate. It might sound like a lot, but when your child starts to get fidgety, especially if they are sharing your seat, a little treat every now and then will keep them still for a few minutes. My daughter usually has a very healthy diet but I decided to let that go for the sake of keeping her happy on the flight!
Drinks for your child are also essential for take off and landing as it will help ease the pressure on their ears. We just took some juice in a couple of lidded beakers and they got through customs okay.
Other essential things to include in your hand luggage are toys, it would be wise to pack a couple of favourite toys and books that you know always entertain your child, these will be valuable items on your journey. We also bought a couple of new books and new toys which kept her captivated for a good length of time, mainly because they were new and exciting. Something we found really valuable was a mini magnetic drawing pad. It was about A5 size and had a plastic pen attached to it with a metal nib, and when you rubbed the pen on the screen, marks would appear which could then be rubbed out. We drew pictures for her, she guessed what they were and then we rubbed them out and repeated the process. This was great because she never seemed to tire of it, it was also an excellent toy for when we actually on holiday and eating out in restaurants. If she was getting bored, this would entertain her while we waited for our food, and it was certainly a lot easier that carrying packets of crayons and colouring books around with you.
Whilst you are waiting around in the airport, it may be wise to keep your buggy or pram with you until you board the flight. We checked ours through with the suitcases and I wished that we had kept it with us, although I didn't know at the time that you could! Our daughter just wanted to be running around everywhere, and airports are very large places and it's easy to lose mobile young children. Luckily we had bought her some reins so we were able to keep hold of her at all times, but it didn't make it any less tiring as one of us was always running around with her as she was just none stop in the two hours we were waiting to board. Strapping her in a buggy would have been a lot easier.
We were also advised to keep her awake for a long time before the flight so she would sleep on the plane this didn't happen as there was no room for her to sleep and she was just too excited to settle down. But this may work for other children, especially if your child falls asleep with a bottle or comforter, or favourite blanket. A sleeping child is a lot less stressful than a wide awake one.
Another thing that we got told was that because we had a young child we would be allowed to board the flight early and would also be provided with extra leg room. However, this did not happen so don't just presume you will get better treatment because you have a child. The airline that we used didn't allow children to sit in the seats with extra leg room for some reason, and they also didn't let children board the plane early. And as it happens, we were in the last group of people to actually get on the plane which was very annoying. So it might be wise to check beforehand what your airline's policy is on children on their flights.
I think if you make sure you are fully prepared before you board the plane then everything should run smoothly. I thought our trip was very successful, even if it was tiring trying to entertain a child constantly for 4 hours. At least she was happy and didn't get upset at any point, and that is the main thing. You know your own child and know what keeps them happy, so I would say even if it goes against your principles, like letting them eat & drink sugary snacks; it is worth doing it just for the sake of a happy child.