Personally, I think it is impossible for a child to learn calligraphy, especially brush calligraphy, without a good first teacher. The child is very emotional. If you let him watch the video for a few minutes, he will be distracted. Moreover, when practicing, it is possible to let him know what is the correct posture of holding a pen, lifting a pen, turning around and swallowing a pen. Watching videos is just an armchair strategist, and real practice is the most important thing. Only when the teacher has been supervising him and correcting him can he keep improving and stick to it. Learning calligraphy is boring, especially for children who don't understand the fun. They will easily get bored and will only give up halfway. But if you just want him to have one more specialty or something, forget it. Calligraphy is not suitable.
But if you insist on using videos to teach your children, you can find them on websites like Tudou or Youku, so that you can choose the one that suits you better.