Once took a recently turned three year old on a two hour flight without too much issue. Obvs at that age an extra year is a huge difference, but he was genuinely loving the take off and once you’re up it’s basically a big ol’ white noise machine. We had a tablet and books handy for him, though he still got restless towards then end
I imagine with yours being younger and the haul being longer it might be more problematic, but I would treat it like any other boring-for-kids situation. Books, activities, hey duggee on the tablet. Hopefully they’ll sleep at some point.
Other than that, just be aware that the people on the plane will be furiously twittering about parents and children and planes. But they’re probably cunts, so fuck ‘em.