Couple of extra things that came to mind as I was chatting with teen DC about our own plans next month.
Make sure you have a local SIM or better yet eSIM. You'll need it for navigation, Grab, Googling.
Allergy and food intolerance awareness is, um, limited. It's very much buyer beware, although most staff will try to be as helpful as possible, in a politely bewildered way. (One of my DC has a nut allergy).
Very close to your KL hotel, all the tourists go to the famous food street Jalan Alor, most of the stalls open afternoon onwards until late at night. It's an exciting bustling place, pretty lights, vast range of international food options and English printed menus with photos so you know what you're ordering. (I can't comment on the food quality though, KL locals almost never go there - I think I've been once in my whole life!) In the daytime, there's great street art in the side roads around Jalan Alor.
Smaller shops and many restaurants don't take credit cards so make sure you have cash.
The mass transit in KL is excellent, clean and cheap (and utterly rammed at rush hour - avoid at this time unless essential). You buy tokens at the ticket machines. I have never, and I mean never, found one where the card payment works - ensure you have plenty of change. You can get change at the information kiosk inside each station. While on the LRT, try to stand right at the front especially going underground between Pasar Seni and Masjid Jamek - it'll be fun for your DS!
The rain, when it comes, is torrential in a way that will destroy your shoes, soak you to the skin in minutes if you have no brolly and bounce up to knee height if you do. And it is warm, and accompanied with terrific flashes of lightning. Rivers of water will pour down the roads and local flash flooding can occur. Very exciting, but I suggest getting under shelter rather than dancing in it.
Finally! The two universal Malaysian gathering points for food and socialising are the kopitiam (Chinese for coffee shop) and the mamak shop (Indian Muslim food). One serves breakfast classics such as kaya toast and soft-boiled eggs plus a huge range of noodles/rice for lunch. The other serves all-day essentials such as roti canai, thosai, another huge range of noodles/rice. These establishments are found in every neighbourhood, so essential are they to our society 
But they are not air-conditioned and might feel overwhelming to visitors as sometimes there's no menu, we just 'know' what to order. So... there are a few chains which have branches all over (just Google) which replicate the setting and food very well, but which have written menus in English, QR ordering, take credit card payment and crucially provide air-con!
- Ali, Muthu & Ah Hock
- Oriental Kopi
- Bungkus Kaw Kaw (pronounced cow-cow)
(Other Malaysians will eye-roll a bit at the lack of local neighbourhood authenticity, but we all go to these chains too!)