They'll be fine! The entertainment systems on the planes have more than enough to keep them entertained, I just did it with a 2.5 yr old solo and survived.
Order the child meals, take some sticker books and extra snacks and load up a USB with some of their favourite shows, or get a portable DVD player. Maybe take their own headphones as the ones provided aren't great.
Check out your layover airport for entertainment, Singapore had playgrounds, theatre, butterfly gardens so is a good one if that's your layover point.