Is it creative as in craft or music? That is a big age range.
If it is craft. I’d probably start with a print out with four guitar outlines on it and get them to come up with some guitar designs. Colouring and drawing is always a good calm activity to do while everyone is arriving. Then bring out large guitars cut out of cardboard with shoulder straps from a choice of ribbons and let them go mad with glue, paint, patterned paper, sequins etc. If you ask around shops you could probably get enough cardboard for free.
You could also design band T shirts, have some famous examples to look at. Then just cheap t-shirts and fabric pens.
At the end you could have a photo booth with other rock star props, music to dance to and they can pose in their t- shirts with their guitars. Either use a Polaroid so they can take a copy with them or send a digital copy to the parents.
Can you get some large rolls of paper then get them to lie down and trace their outlines, then paint/glue/collage their rock star alter egos? Have a naming session. Play some fab rock music - do musical statues/ best dance off as a filler activity.
Rock star hangman; rock star wordsearch; rock star charades (though as someone said, that's a heck of an age range so I could see how they might be too old for the younger ones); rock star "mood boards", cardboard and old magazines that they can cut and stick (will the littlies have someone to do any scissor work for them?); your local charity shop might have magazines you could pick up cheaply.
And I know it's not a voice workshop, but what about a few minutes on breathing and/or a Do-Re-Mi type exercise? Also, years ago when I did a voice training workshop one of the things they did was get us to project our voices against different surfaces e.g. curtains versus hard walls. It makes a huge difference to the sound. And it might sound elementary, but so does standing up straight and making sure your mouth is open and making the right shapes for vowel sounds...maybe some mirror work so they can see the difference, or pair work?
And depending on what kit you'll have with you, there is free software out there (Audacity springs to mind for laptops, not sure if they have an app) that allows you to record your voice and add different sound effects. Depends on how many kids you will have, though, and how many helpers/how comfortable they are with techy things...Good luck!