I must admit that I would be rather leery of "UK models". I am unimpressed with the professionalism displayed on their site, and those links to the Mirror experience provided by shoota suggest they are far more concerned with attempting to get you to part with some serious dosh on the day of the shoot by convincing you that a portfolio is needed.
As I understand these things, a portfolio is not needed - at least, not by the top, reputable agencies. Were I in your shoes, pixie, I would be aiming to persuade my child to submit some snaps of herself to one of the agencies on this list (AMA) instead. Agencies like Storm, and Model 1, both of whom are insistent that a portfolio is not required.
http://www.associationofmodelagents.org/