Do they live in Italy or in Britain?
If they live in Britain, the answer's easy: send her to kindergarten (when she's old enough), and she'll pick up English the natural way, and very quickly. By the time she's in school, she should be more or less bilingual.
If they live in Italy, it really does depend on the child. The general rule of thumb is that young children learn languages much more easily than older children, although it varies greatly from child to child. As a general rule of thumb, it starts getting harder after puberty.
Part of the problem is that very young children will learn at their own pace, and some will refuse to learn at all.
I was once brought in to tutor a very bright four-year-old. The idea was that I would only ever speak English to her. It was to be money for old rope: I'd arrive, play with her for an hour and a half and get paid for it. Unfortunately, she was bright enough to work out exactly what I was trying to do and she thought it was a very mean trick, so that put paid to that idea.