Ok, for anyone reading this years later:
Don't sweat it guys. We are talking about freaking kids here - some will be learning to use the materials for the first time and find that they are just not their bag. Use The Works. It's cheap. You can buy mostly everything you need online. Free delivery. Crawford & Black and Reeves brands are good enough. They are student quality. And your kids are.... students! If you wanna splash out, there are higher pigment standard Liquitex paints which are getting on for pro quality that The Works sells as well. WHSmith is overpriced, lots of 'real' artists don't end up in the art shops either... that's more for the Sunday Painters. Mainly because the materials are so costly and you can source the larger quantities often needed elsewhere if that is your livelihood.
I'll let you in on a little secret. If you want to do a Fine Art degree, it won't be prescribed like GCSE's are. No one tells you what materials you should use and you don't have the pressure of drawing realist work that you take home and satisfy parents, family & family friends with (unless realist work is what your child wants to do).
Like most students, when I was navigating my degree, I was strapped for cash - and . So, it was Crawford & Black paints for some projects. I even experimented with using children's arts materials (it was relevant to what I was doing at the time). By the time I got on for my final assessment, I needed to use paint in huge quantities, I using tubs of decorating, floor and enamel paints.
I got a First Class Degree in Fine Art.
Oh, and a little tip on the brushes: the bristle ones in The Works are crap. Maybe head to The Range for those, they have actual artists ones. I really like the soft/smooth white plastic bristle ones at The Works, the brown fake sable ones are tolerable. Even though the white bristle ones are at the lower end of budget brushes, I like the feel of them across all paint mediums (esp acrylic) for the style I want to use, and you cry less if you wreck the head by letting paint dry on it, etc (it's easily done). Sable brushes are beautiful to use, you can buy them in different levels of quality - again try The Range. Quality brushes you select individually are expensive - even getting a selection of lower budget ones aren't cheap. Make are they are thoroughly cleaned because if you have any of these and you lose them to dried paint. Easily done.
Good luck... and if the art teacher complains remind them that you are more likely to find artists shopping in their local DIY warehouse for materials, locally it's been gardening spaces, knitting and even work involving a barge recently. That's what the Arts Council is funding, anyway.
Hope that helps... I get kinda ranty when I see long lists of materials, hours of homework, etc. It puts unnecessary pressure on kids & parents.