The most useful is 1cm square grid. If you have trouble finding some, ask the local school for one sheet and photocopy it. Obviously, use only 'dead-headed' matches. If you can manage it, buy some 'click-together' plastic cubes. These are available from educational suppliers and some toy stores.
Alternatives are small wooden cubes and sugar cubes.
Other useful items . Useful, but not essential, are a pack of ordinary playing cards and a set of dominoes. A calculator and the usually assortment of pencils, scissors and paper will be useful. If you think that your child is very able, gifted or has special talent, perhaps you'd like to contact an organisation that has expertise in assessing such children and advising families and schools about catering for their needs. The NAGC has branches across the UK, many of which run Explorers Clubs for children.