I love making bubble bath and body wash with my craft groups. It's such fun to see what colour and fragrance combinations they come up with, and they're always so thrilled to make something that's their own.
If you are crafting with younger kids, you can make the base in advance and let them add the scents and colours. If you're crafting with older kids, make it with them! I always make the recipes in volume because I don't have 15 scales, but if you have a small group, weighing the ingredients is always a better way to go for accuracy!
As an aside, I find some of the kids in my groups have issues with the idea of preservatives. I use this as a learning opportunity to explain the value of preservatives and why we use them. I also tell them that no one leaves the room without preservative in their products as I don't know what is going to happen once they leave. I have one young woman who proudly told me she still has a bit of her body wash from July 2007! EEK! Make sure you get the kids to write down an expiry date of no more than one year from today's date!
Don't offer more than 4 or 5 fragrance options - they can take all day to make choices, and they'll want to make all of them, which can be an issue if you're on a limited budget like I am with the craft groups. I find something like Pink Sugar is very popular, as is vanilla and something fruity, like strawberry or peach. If these are intended as presents for adults, bring along something like Black Amber Lavender or something a little "darker" and encourage them to make at least one of those for a more adult present. (Although if your child made you a bright pink Pink Sugar body wash, you know you're going to use it!)
For the colouring of the products, make sure you tell them how much to use - I always tell them to start with one drop and work up to more. And no, combining red and green won't give you a red and green bubble bath, it is going to give you a brownish thing that looks just awful!
I always find it's best to make a few small bottles than one big bottle. I try to have at least 3 - 2 oz (60 ml) bottles per crafter so they can play with colours and scents! With youth, more is always better than less! And encourage them not to bottle things if they don't like the look of the product! And keep an eye on the bottles - if you've established you're making 3 bottles, there will always be one kid who wants to make 8 and will find a way to get more bottles out of you!!! Those things are expensive!
Oh, and make sure you have a storage container for whatever's left over. I always have a cup or two after the class, and it's a pity to waste it!
Make sure you have tons of funnels on hand to fill the bottles, and ensure you have paper towels or a disposable table cloth on the table! There will be a mess - but it's a good mess!
Use disposable items - cups, stir sticks, spoons, pipettes - so you won't have a huge mess to clean up. Yeah, I know it's not environmentally friendly, but if I had to clean every item after a craft group, I'd have the energy to offer one per year!
Get your labels ready! I make sure I have a ton of labels and some Sharpies at the ready and I encourage them to change the fragrance name and make it their own. I love some of the names they create - Bobbi's Bubble Bonanza or Fiona's Fluffy Fun - although they don't have to be alliterative. Oh, and make sure you have the rubbing alcohol ready in a spritzer bottle so they can clean the sides of the bottle before adhering the labels. (If you want to make water proof labels, I like the "Let It Shine" spray from Krylon. Spray on the label, wait about 20 minutes, then adhere to the bottle. It'll keep the felt from running!)
And have some cute packaging like cellophane bags and ribbons so they can make little kits with a soap or a candle or something similar. If they are making body wash, check out the dollar store for the 4 for $1 body scrubbie pouffie things they can add to the kit.
Have fun! You will get messy!