Give your child a watch and read the time together
Make sure the child is with a friend of similar age whose parents agree to you trying out the trust technique
Tell them they can go out to play if they come back in 15 minutes
If they return in 15 minutes, tell them they can go out for another 20, showing them you trust them to return on time
Build up the time you allow your child out on their own, letting them know that everytime they come back on time, you'll reward them with more trust in their next outing.
