I've found a really great series of books to introduce grammar. Its called Words are CATegorical and uses silly rhymes and cartoon cats to teach the basic ideas of verbs, adjectives, nouns and adverbs.
They are short, simple and comprehensive - a great introduction for older kids that could probably be used with toddlers, too. The individual titles are:
Dearly, Nearly, Insincerely - What is an Adverb?
To Root, to Toot, to Parachute - What is a Verb?
Hairy, Scary, Ordinary - What is an Adjective?
A Cat, a Bat, your Grandma's Hat - What is a Noun?
Anyone in the UK can get the set for £5 from thebookpeople.co.uk
That's about 80% off.