First, your third grader needs to be comfortable with place values (there are some great worksheets on this site to help with that).
Rounding to the nearest 10 requires you to be comfortable counting by 10s. The best way to illustrate rounding is to draw the number line on a piece of paper - from 1 to 40 - (see link) and then draw a hill over the top where the edges of the hill touch 20 and 30 (the hill is between 20 and 30).
Tell your child to imagine that they have placed a ball on the hill. The ball rolls down the side and ends up either on the 20 or 30 depending upon which number you are rounding from. Example: The number you are rounding from is 26 to the nearest 10. The imaginary ball is dropped on the 26 and rolls down the hill to the nearest 10, which is 30.
If the rounding number is in the middle, the child can visualize that the ball sits on the flat top of the hill (25 in our example) and your child can push it either way (to 20 or 30).
A graphic example usually works best with these types of conceptual problems.