Where are you going in Malaysia?
We've been going there almost every year since our twins were toddlers to visit my parents (I'm Malaysian, obvs), we are old hands at this, they're late teens now!
Main long haul tip for small kids is - all screen time restrictions are lifted! On long haul flights we let DC watch all the kids' shows and play all the games they wanted on the IFE until their eyes went square. The sheer novelty of being allowed such wonders kept them remarkably quiet for most of the flight.
Another niche tip for flying to Malaysia from the west - there's almost always turbulence across the Bay of Bengal, which takes around 2 hours to get across, often seatbelt restrictions will be on for big chunks of this. We still have a family 'policy' that everybody has to go to the loo, even if they think they don't need it, just before the plane reaches the Bay. <bitter experience>.
When you get there - as @notenoughtogoround says English is widely spoken to at least basic communication level, most Malaysians adore children and will make a fuss over them. There's loads of culture, activities, etc.
Child car seats are still not widely used and not routinely available in taxis/hire cars. When they were little DC carried their own Trunki car seat backpacks as hand luggage.
Do not drink the tap water anywhere, even in hotels. Always drink either bottled/packet/tin drinks, or take cooled boiled water. Every hotel/apartment I've ever stayed in has provided either free bottled water or a water dispenser. Similarly, especially for a small child, beware of ice in drinks (except in hotels where it's always been fine for us). It is less of a thing these days but several decades ago when DH first visited, he got awful stomach flu probably from ice - back then most local shops/stalls would have just frozen their own ice from tap water.
Always be aware of the equatorial sun and how intense it is, especially to young skin. You'll notice that Malaysians stay out of the sun, cover up or use an umbrella (!) to shade themselves. So especially by the beach or pool, make sure everybody is SPFed to the max, have a big parasol over your sun lounger, do not swim in unshaded areas during the brightest hours of the day. The beauty of the tropical heat is that you can go on the beach/in the pool from dawn to 10am, then from 5pm to midnight if you want, it will never be too cold.
Also re: beach, all the Malaysians will be on the beach in the evening, 5pm onwards. That's the best time because the sea breeze will have got properly going, the sun isn't as scorching, you can watch the sunset.
Mosquitoes are £&$# &£%@!! Most bite in the evening, which is annoying because that's when you want to be outside enjoying the slightly cooler temperatures. But the little whining £*&^$# can strike at any time. Slather yourself in repellent! Have after-bite soothing gel to dissuade scratching.
Malaysians don't have separate early teatime, or early bedtime for children. Even on a weeknight you'll see whole families out and about in shopping malls, night markets, just for a stroll because it's cooler. Kids in pyjamas sometimes! All eating dinner together. Just roll with it and forget home routines.
Most important - the food is A.MA.ZING. Don't miss out on trying all of it! Even the dodgiest looking hawker stall down a back alley will be amazing and safe to eat - just check it's freshly cooked to order. Malaysian food is often spicy, but with made-to-order things, they'll be happy to omit the chillies for children.