First of all, 5 and 6 year old children often cannot pronounce certain sounds very well. I would ask the teacher whether their speech is within the normal range of other students they teach. If their teacher says they are not speaking as well as they should, your child might need to see a speech-language specialist. Second, stand behind your child and speak softly just to make sure they can hear well. If you suspect their hearing is poor, they should see an audiologist. Third, language specialists teach children how their mouth is shaped and where their tongue is placed when they are saying the different sounds. You might try showing them that as you work on specific sounds.