Hi - ex-TA (male) here :
As your post has been ignored all day I thought I'd reply to you!
I don't know if art can really be 'taught' if a person has no interest or aptitude for it. I was never much good at ordinary drawing or painting, but when I was older I was VERY good at technical drawing, with rulers, compasses etc, and I did even wonder about trying to be an architect, but that idea didn't last long.
You don't say if they are boys, girls or one of each (not that it makes much difference really.) Also when I was primary age I went in for a 'gummed paper craft' competition, which involved tracing shapes onto the back of the appropriate colour gummed paper, cutting it out and sticking it down in the correct part of the picture; I won a book for my effort.
As your children will be transferring to secondary in a year or two, it would be a minor advantage if they can draw or copy still-life items; I remember I had to draw things like a half-open matchbox, and for that a knowledge of perspective and distance are required. Our DS at grammar school had to take his tie off and draw it. You also mentioned music : do they play any instruments, as that is a useful skill and good for social reasons, as well as creative.
Of course, these days a lot of art work can be done on the computer, either with a vector DRAWING prog, my favourite being : www.xara.com/uk/ but there are cheaper or even free children's progs, or a PAINTING prog which probably came with your computer software.
Looking at the work of famous artists may give a glimmer of stimulation to your DCs, and there is masses of that on the web. Some artists make millions from abstract paintings, which might look like they have been done by a child, but it must be having the original ideas that is the important part. Back in the '70s I went to a Bridget Riley exhibition and I love her work, which looks like it's been done by computer, but hasn't : www.op-art.co.uk/op-art-gallery/bridget-riley
Books can teach the basics of drawing and painting : shape, perspective, texture, shading etc, and quality pencils HB, B, 2B, 3B or softer, are nice to use. Painting can be watercolours, oils or there are modern acrylics, and may be others I don't know about. Colour pencils or pastels are not too expensive, and can be enjoyable to use.
Being brave and 'having a go' is probably they best thing they could do; don't worry about the results, but just create something different and new, and see what turns out!
Good luck.