I am preparing a mathematics syllabus for students, so I need to give a heading like "Adding and subtracting small numbers OR Addition and Subtraction of Smaller Numbers." which one is the correct one ?
They're both correct.
Adding = Addition
Subtracting = Subtraction
The difference between the two is that
addition is a noun, whereas
adding is the present progressive form (gerund) of the verb
Addition and Subtraction sounds a bit more scholarly than Adding and Subtracting does. But it is not more scholarly, really.
You are changing two separate things in those statements. The add/subtract change does not affect the meaning at all--both statements are the same--but "small" would be the more typical word vs. "smaller," unless you are specifically referring back to a previous lesson. There is nothing about the add/subtract changes that has any bearing on small vs. smaller.