This advice below found online is relevant. Bear in mind also children can usually only focus on a single activity for their age + 2 mins, so don't expect more than about 7 minutes on any one thing. Not to say they can't go longer if they are really engaged :)
"Children's cognitive, linguistic, and motor-skill levels also affect their willingness and ability to concentrate. If an activity is too challenging in any one of these areas, children either may choose not to participate or may stay with the activity only for a short time. For example, children who choose blocks over art tend to feel more confident in and comfortable with large-motor skills than small-motor skills. Our role as teachers is to support children in developing concentration for activities of their choosing (by providing ample time for them to choose each day). At the same time, we need to gently encourage children to experiment and stay with activities that challenge skills they're not as comfortable with (by providing entry-level activities that are both inviting and potentially successful).
As you know, children's moods also have an effect on their ability to focus. If a child comes to school upset, tired, or overly excited, he may be too distracted to concentrate on an activity, particularly a new or challenging one. By understanding that his lack of concentration is related to a mood, you can help him deal with the cause (the mood), not the symptom (the lack of focus). Once the cause has been sensitively addressed, the symptom just may improve."