Specsavers! Got mine last year for £25 and they have changed my life. No idea if they do kids ones but the adult ones are fab. I'm really really shortsighted with astigmatism. They only correct the distance not the astigmatism but they mean I can see the clock, my locker number without head butting it and which changing rooms are which (yes I have been in the men's by mistake!). Anti fog coating is good too. Highly recommended.
I saw this thread last night and thought I'd order some for myself, but I couldn't find my prescription. Went into Tesco's opticians this morning to get my prescription and came out with prescription swimming goggles for £9! Now that's a result I thought was worth sharing!