What grade is your child in and in what areas are they experiencing learning loss?
Here is what I would recommend:
Identify their strengths and weaknesses. Work to draw upon their strengths as you build up their weaknesses.
Focus less on memorization and more on conceptual understanding. Elementary is the foundation, so now is the time to make sure that they are really understanding math, not just going through the motions.
Make math fun! Consider exploring games and puzzles that incorporate mathematical reasoning.
The key to catching up is to see where the student is currently in their Math skills. Sometimes going back to the basics is what is needed in order to move forward. Emphasizing basic skills and increasing student self-confidence would be a focus as well. Giving students real-world problems will help them to see Math as a subject that is everywhere and that having a good skill set in Math will take them places in the future!