If my child who's above 18years ask me about helping him in betting in soccer I will help him ,to me there's nothing wrong with it we as human being has the free will to do what ever we want so it will be against your child's interest for you not to tell him or her.
The highest you can do is to give advice and make the child see reason why he should not engage in betting as it can become addictive and become a problem in the future