For ages 4-6 I recommend the Longman series of books. They're mostly well structured aside from the dreaded Longman 4!
For ages 7-10 I recommend a series a books called SPEC. We use them at my school and they're very well set out. However, I think they might be specifically designed for our chain of schools so I'm not sure if they're commercially available.
For teenagers, I'm not entirely sure - you could try LiveWire. For teenagers you can try using NEF or Cutting Edge but you'll need to supplement a lot to bring things more to their level of English and interests.
Hope that helps.