fabricland (southern) and Abakhan (northern) are fairly cheap, both have websites, although you have to order by phone on fabricland and they charge you the actual Royal Mail postage, rather than a flatrate like other places. Also try Croft Mill - I love the little descriptions of the fabrics, someone at the mill has a great sense of humour.
otherwise, there are so many fantastic quilting fabrics that would make fab children's clothes. i think in the past quilting cotton was inferior to dresssmaking cotton but know it is excellent quality and suitable for clothing. the only problem is that they are around £12m!!!!
Try fabricrehab, backstitch, m is for make, fabric loft, raystitch, kitchycoo Dots and stripeswhich also sells ottobre magazine, it is full of seasonal patterns for children from 0-16. It is supposed to be really good but I have never bought it because I can never make up my mind which to buy. For the number of patterns you get, it is really good value for money
I have the book sewing for boys, and I have managed a couple of the projects by upcycling charity shop buys. I have boys so find it much harder to buy cheapish fabrics for them.
I have also made an oliver & s pattern - they are really fab & great for all skill levels, even a beginner. They are very expensive however.