Every tongue twister helps to work out this or that sound. But some sounds seem very hard to pronounce for the learners of English, especially younger pupils. This can be the visual aid for training [θ] and [ð] sounds. From my observation I can say that it's hard to obtain good pronunciation of the combination: [θs] and [ðs]. I hope this exercise will help you to do it and also will bring fun for your pupils!