Younger students can build on the letter decoding skills included in tongue twisters by reading phonics poems in books such as the Phonics Through Poetry series. These poems are a little different than traditional tongue twisters, but they are a fun way to practice certain beginning soungs, rhymes, digraphs, and more. You may also want to discuss what makes these sentences and phrases so difficult to pronounce quickly.
To build in writing practice, the students will have a blast building their own tongue twisters. To start, you can have the kids make four columns on their papers: one for adjectives, one for nouns, one for verbs, and one for other parts of sppech. To determine the letter for their twisters, I usually just have them pick one of their initials. This gives them a little bit of free choice, but also ensures that you don't get 20 twisters of the same letter.