My sister has now got phones for all 3 of her children (11, 10 and nearly 9). The oldest two have had them for a couple of years and she has found they don't really care about the actual phone. They all have ones that they can take pictures on (all be it low quality) and they can all have music put on them too, which the kids love. She has made sure the "ugrades" they have got all have slots for memory cards so you can make sure the music is on there.
There are a lot of good little phones in Tesco and they also do a great offer with credit were if you top up (I think) £10 a month, Tesco will give you another £20 credit for free. Tesco also have their own download shop for ringtones and games which Sis has found priceless. All 3 kids have at some stage lost all their credit by downloading a game or song from somewhere else and it's wiped their credit and sometimes not even got what they ordered.