As well as books, it's the practical fun side of maths that will help most. Baking together - weighing and measuring quantities, repeat with water plants (using a jug), creating height and weight charts for each other, using real money - plan a tea party (handle the money) and of course there is a multitude of boardgames that are great for maths. My DS learnt numbers 1-100 playing Snakes and Ladders in the Summer holidays before entering reception. I like the Orchard Toys for educational games and the Puzzle Company also have some good things. I have also used various Internet games/sites to reinforce maths skills/concepts, particularly Education City and BBC Bitesize. Have fun with them - they learn better.