For some children, it can be difficult to sit still for an extended period of time, which is why I sometimes suggest to my students that they keep a small piece of blu-tack, an elastic band or even a small piece of paper and pen with them, which they can use to fiddle with when they need to.
It may sound strange, but actually research has shown that "one way that children cope with distractions is to give their attention a mindless task, like fidgeting and fiddling…allowing the rest of their working memory to get on with task at hand, learning, uninterrupted" (Smith, 2010).