Just like what you say is like a person, what you want to change is not his words but people. Many people can't write well when they are young, but when they grow up, they can write well, which means they are mature and steady. What you require is the height of calligraphy, which also requires a process and people. Not everyone can practice writing. If you are an active child, you might as well learn Taekwondo.
The most taboo for parents is to completely add their own life path and understanding of life to their children. Therefore, you should lower your expectations, give more guidance in character and life, and don't put too much pressure on your children when practicing calligraphy. That's a lifetime thing. Sitting posture, set an example, often sit up straight in front of children, reading and writing.