Thanks for all the replies and I will have a look at all of the tutorial links posted by EvilNeko and Angelaz.
In response to trooper's post, I have let the children experiment with the program within the last few weeks and I am now looking to use paint.net to create something that will show the children how easy/simple graphics editing is.
Also in response to Bob's comments, which plugin do you suggest I use to make the tattoo effect more realistic?