Here we have a set of worksheets which looks at subtraction of whole tens. Once children have become confident with counting forwards and backwards in ones and twos the next step is to be able to count in tens. Be careful, however, as some children can rote learn this with no understanding of how large the numbers are, so it is a good idea to have practical apparatus handy.
When confident with counting in tens they can be introduced to addition and subtraction of whole tens. A number line is useful to begin with, so that they can hop up and down the line saying each whole ten in turn or marking with an arrow each step of 10. Eventually they should be able to do these without the help of the number line.