Can children understand a speed of a car?

Yes, smart children who are familiar with cars can understand this.

No, unintelligent children who are not familiar with cars cannot understand, at least not immediately.

The level of understanding can range from the relative measure of fast versus slow to the more subtle measure of units of distance per units of time. The simpler, real-life level of understanding involving the experience of moving in a car is achieved by more children than the abstract understanding involving measures of relative units.

This is also true for adults.

However, on a deeper level, who among us truly, deeply understands the nature of speed, motion, and time, in all its implications of subjectivity, objectivity, and relativity? Even Einstein was still searching to understand this more completely.