Children need plenty of practice at counting on and back, not just in ones, but in twos, threes, fives and tens. It is also important to start at different numbers.
These four maths worksheets concentrate on counting up and down using number tracks or number lines with missing numbers. The more confident children are with this the easier they will find adding and subtracting 1-digit numbers. A key part of this is working out what the rule for the sequence is eg adding 3.
These pages can be found in the Superbrains section, under Counting and Number.