Hi, I'm no expert on this but I really can't see why her 12 year old can't join with her - after all it's done under parental supervision - we have a school girl that comes in to get weighed in my class and her mum comes along with her.
The diet is so easy! On a red day - you can basically eat as much meat (ham, chicken, beef, lamb, fish etc) as you want, providing any visible fat has been removed and it's not cooked in fat! A green day is pasta and potatoes (jacket spuds are fab and fill you up - and you can have a tin of beans on it too!!) Most, if not all, fruit and veg are free so you can eat them until you are full. You can beat as many muller lite yoghurts as you want (tesco have a cheaper "like for like" yoghurt). To be honest, about the only thing I ever measure out is my cereal in the morning and my milk allowance for the day. You can have what are known as healthy B options - which will allow you to have a certain amount of potato or pasta mixed in with your red day but until you've read the book, this is very difficult to explain.
Basically, as fab healthy diet for all to do with virtually no weighing!
Hope that does the job and answers any questions.
Love
loopyloo
xx