Below are some useful sites with English resources which can help children and teenagers learning about language and grammar. They cover literature, poetry (reading and writing), and grammar. Some include word games that will appeal to some children.
Age recommendations vary enormously depending on the ability and interest of the child. If you want a very basic introduction to grammar and parts of speech, see the page introducing grammar.
BBC Bitsize English – brief descriptions, interactive activities and exercises in reading, writing and grammar. Officially for ages 8-11 (primary Key Stage 2) but levels vary between activities.
Reading matters – a useful selection of book reviews for children (about age 8-11) and teenagers. Information about authors and suggestions for further reading too.
Teach your monsters to read – an award-winning site which offers three levels of games intended to teach and reinforce understanding of phonics. The simplest is for young beginners; the most advanced for children who can read simple sentences and have a good understanding of basic phonemes.
Poetry 4 Kids – rhyming dictionary, poetry games, poetry writing contests, and how to write a humorous poem. For any age.
Poetry Writing – helpful introductions to poetic forms and tips for writing poetry
Fun Brain – word games to help children appreciate word differences. For primary age, depending on ability.
Learn 4 Good – actually intended for people learning English as a second language, but with a useful set of grammar rules and explanations for various forms of speech. For teenagers (and adults!)
Blue Book of Grammar – a useful set of rules and examples for some of the trickier grammar questions. Mainly for teenagers.
Guide to Grammar and Style – for older teenagers who know the basic rules and want to learn ways of improving their writing style. A miscellany of rules, explanations, comments and suggestions.
Humorous Grammar Rules – for those who basically know how English is conventionally written, this is an amusing list of rules, each one breaking itself.
Absolute Write – excellent resource for freelance writers, with market listings, articles and interviews. Mostly adult writers, but this would be suitable for interested teenagers too.
English National Curriculum (key stage 3) – for anyone wanting to know how English works in British schools, here is the outline National Curriculum for English at Key Stage 3 (approx age 11-14). You can navigate around the site to find similar outlines for the other key stages.