Would your audience enjoy being part of a competition? A competition should be fun. It should not involve money. It can have a prize or many prizes, but the prizes could be awards and certificates.
The competition should directly relate to what you want to communicate to your audience. Busy people will only want to spend a minute involved in a competition. If it is a competition between classes or schools, it could take three of four lessons to design something.
Do not have a competition that could have many people giving the same answer. Think of lots of ideas and design something new and interesting.
Having a competition gives you an excuse to let everyone know the winner and to tell them something new about the issues you are communicating.