This is a much trickier exercise than it first appears. Firstly, children need to be confident with reading large numbers and counting in thousands. Secondly they need to be able to visualise marks on a scale.
Only the zero and 10 000 are marked on the number line so it would be a good idea to jot down each of the thousands before trying to estimate where the arrows are pointing.
Then it is a good idea to work out if the arrow is pointing above or below the half way mark and then make an intelligent estimate according to this.