When a child has indicated that they want something, this is an excellent opportunity to practice waiting. A child can learn to wait for extended periods of time especially when using their highest reinforcer to motivate them to learn to wait. For a very small child, they could wait for 2 seconds as a starting point. For an older child it could be 15 seconds. Thereafter, we slowly increase the time they are expected to wait.
We want the child to hear the word WAIT and then to wait appropriately. The child will begin to associate waiting when they hear the word WAIT. The more practice they get, the more successful they will become at waiting.
If the child can do that in a short space of time then they can have access to the reinforcer they requested.
Teaching a child to inhibit their desire to run across the road or grab something off the table or stare out the window is possible. Inhibition is a skill that is required in order for a child to be able to wait.