Looking for a way to explain rounding to a third grader. The child gets the 0-4 round down and 5-9 round up, but for example 549 rounded to the nearest 100 I can't get that the number is between 500 and 600. Any suggestions would be appreciated.

549 rounded to the nearest 100.

Where is the 100 part of the number? That is the third place to the left (count the 100 and that's three places). So the 5 is the only number that counts. It must be 500 (if we throw away the 49) or 600 if we round it up. Hope this helps.