The Mathsblog section on year 6 maths worksheets, Understanding Number, has a growing number of worksheets covering a wide range of topics. The work expected in Year 6 is very challenging as numbers are used in ever increasing and decreasing sizes. Rounding numbers right up to millions is included, together with rounding decimals and estimating and approximating numbers.
There is also some excellent pages on fractions, as well as relating fractions, percentages and decimals. Prime factors are also introduced. Keep an eye out on this section as it is updated with new worksheets being added on a regular basis.
For a lot of Year 6 children this work will prove too tricky and I would advise looking at some of the earlier year groups for suitable material, in particular year 4 and 5.