By year 6 children should be able to read and write very large numbers, up to millions, as well as negative numbers. Many, however, find this very challenging, as indeed do many adults.
These two free maths worksheets for year 6 are much harder than they might at first appear. The number lines only have starting and finishing numbers so children have to work out the intervals. In some they are in intervals of 1000. For the negative numbers there are no intervals, making estimating even harder. With these it is probably best to work out what the half way mark would be by finding the difference between the lower and higher number (eg the difference between -80 and + 20 is 100 so the middle interval would be -80 + 50 which is -30).