Giving the choice between Chiang mai, ( where you could easily spend 1 week and have a great time renting a car an visiting lots of places and see lots.
vs Penang, (i assume u mean Georgetown, which i love) only, as the main land is nothing to see, 2 days is plenty to take in all the sites and nothing really a toodler would find interesting or exciting other than the tram ride up Penang Hill
Langkawi offers not much compared to phuket, outside of self contained resorts.
KL; is the only city in SE asia i went an didn't really care for and out of 35 years here, never went back
Obviously you now see there is not one answer on where to go an spend your time.
Good luck
Phuket ranked #6 in top destinations .
https://thethaiger.com/hot-news/phuk...d-trip-advisor