A small child understands what 10 minutes is if he is told "when the sand ends".
Please choose the according video here, tell to your child "You can play with your cars untill the sand ends and then you will brush your teeth and go to bed".
This works brilliant with my son, he is 3 years old. In the past, he protested against any time limits, because he didn't understand what 10 or 15 minutes means. Now he grabs his tooth brush, when the sand ends.
Videos with relevant time limits are placed on this website for you and your child.
Here is a stopwatch for kids. Its a sand timer which small children understand well.