I got pens from elc and put three for each child in the party bag after the party. I used baseball caps as I got them for £1 each, I got lilac, pink and beige ones which looked lovely when coloured in. The children did it at a table. Look in Tesco/asda for t-shirts though it can be hard to buy a few in the same size which is why we went for caps.
Asda blue or pink t-shirts 75p each.
Go for paints, at that age they can manage them. Pens are rubbish really as the material gets rucked up, even with 2 extra people stretching it and you just get bike tracks!
Split the kids into teams so they can sit at a table to do it. Let the last team be the kids who live closest so if their t-shirts aren't dry at the end you can just pop them round the next day. Hang a washing line indoors if it's raining and stick the heating on to help them dry.
OK. T-shirts from Primark. Pens for drawing on t-shirts from Hobbycraft. Not very good imo, but they are only 6. I use Pebeo Styli Stick pens, which are also in Hobby craft. They are like squeezy pens. Can get quite messy, since 6 year olds love to squeeze! It takes a couple of hours to dry though, so probably better to use the pens which are just like markers. If you can get to a Hobby craft, you could also get some iron on motiffs or iron on ribbon to tart them up a bit. Also you will need a piece of card to go between the t-shirts to stop the colour bleeding through to the back. Table is better to work at if you use the paints, floor is fine for the pens. Anything else?
Thanks for all the info! I'd prefer to do it on the floor as we're having it at home. I'm a bit concerned about the pens being rubbish but DD is one of the oldest there so everyone else will be 5. Maybe I could supplement them with gems etc and some glue??