We own the box-set of Thunderbird episodes and can report that these stories are all simplified versions of existing episodes. This is rather useful when reading to 5 year old who love to hear / watch the same thing endlessly.
We have owned this book for over a year now and it has withstood "reading" by a 3 and 5 year old. As the stories are quite long and have relatively complicated Thunderbird descriptions in them, I think you would need to be a competent reader to handle them yourself.
The pictures are hand drawn, rather than stills, which disappointed me but does not bother them. Good for young fans.