The best may be to get specific guidance from a speech therapist. Your school district can provide you with one to evalute the child. Even if the child is not eligible for therapy, ask the therapist for ideas and activities to help the child.
In my opinion, you should help them by calling on them often to answer questions, and you should buy a box of candies or treats to give to whomever answers the question properly and loudly. Another choice you can choose to do is to take them to speech classes. I hope this help with your situation. Good luck and have a wonderful week!