I strongly recommend meditation! I’ve been doing 10 mins a day for the last 8 months or so, and my attention has greatly improved!!! I’m using an app called Waking Up by Sam Harris, and the app has meditation lessons specially created for children. I also recommend neurofeedback training, although the objective of meditation and neurofeedback is the same, except meditation is cheaper, and doesn’t require a computer. I tried both and liked both.