Hi, we flew with Singapore airlines at Xmas time with a 3 1/2 year old. There's no need to buy kids head phones, they have special kids ones plus they're different; they have 2 connectors not one so normal headphones are no good.
Definitely book children's meals; they come with a little lunch box full of snacks and juice to save for later. Also, bring plenty of your own snacks as my son ( normally a brilliant eater) refused to eat any of the plane food and on the first keg, our agent had forgotten to book a children's meal so he had to have an adults meal which were very rich and included ice cream with alcohol in. Singapore airlines were very apologetic and sorted it for the next leg right away, but there was nothing they could do at 3000fft in the air!
The entertainment was good, there was Disney Jr on on repeat and some good kids films like Cars 2, Madagascar (whatever number they're in now) and Ice Age (ditto), so your kids can be sure to find something familiar.
We were really lucky, Oli slept for almost all of the flights, he was asleep as we went down the runway at one point! But if your child does cry please don't spend hours 'shhhhhing' them at the top of your voice, it's way more annoying than a crying child! And after hours of shhhhing definitely don't start shhhhhing actual songs lol.
Oh and my best advice. Don't buy Trunkis, especially on your own with 2 children as they WILL want to ride on them all of the time, then they don't get the exercise benefit of walking a d the damn things just don't corner! They're ok when it's quiet but when there's other people around its difficult not to bash their legs.