You probably have somewhere to stay on langkawi already? But if not then Bon Ton resort is lovely- traditional Malay houses in a peaceful location. The restaurant is excellent too, and worth a visit even if staying elsewhere.
Penang is a short flight away (air asia are good for internal flights) and great for temples and easily accessible local culture- you could happily spend up to a week there- chinatown, little india, tropical spice gardens and penang hill all recommended. straits collection is a great place to stay, right in the heart of the old town, it's apartments with cafe attached, with very friendly and helpful staff.
For an island or beach resort I'd look at tioman or sibu (sea gypsey resort? basic but child friendly)on the east coast or the perhantian islands further north. Or aryani near terengganu? (Or tanjung jara, south of terengganu, but that'd be more expensive)
Other places to consider could include the Cameron highlands (tea plantations and cooler air), Melaka (touristy and becoming a bit spoilt) and definitely KL.
For jungle I'd possibly think about a trip across to Sabah on Borneo, if you have the time? It'd be a 2 hour flight, the Kinabatangan river or Danum valley are amazing- orang utans, gibbons, monkeys and crocodiles... Borneo eco tours do good packages. Back on the peninsula Taman Negara would be an option.
Hope this helps a bit- the above hotels all have good websites- let me know if you'd like any more info. I live in malaysia and have travelled around a fair bit- but not with children- but it always seems very child friendly and I think you'd have a great trip.