Just saw an email from the homeschooling group about a trek up Gunung Lambak, a 510m high hillock near the town of Kluang, Johor. Apparently a popular day trip for families, the trail apparently is clearly marked, has large steps and rope bridges on the way and is said to be suitable for 5 year-olds and above. You can book the trail here through a local agent. Other Malaysian outdoors options here. Also stumbled up on a Youtube video taken by a Malaysian family going up. Cute!