By now, your child should be able to count up to the hundreds. If they're still learning the tens, that's okay - as long as they're going at their own pace. This worksheet will be easy if they can count to ten. Ask them to trace the lines and skip count by 5's.
To do this worksheet, your child needs to be able to measure accurately, add and subtract. They must measure both items and subtract the shorter from the longer to work out the difference in length.