Is it a good practise to involve a child in a statement of purpose? Is it a good practise to involve a child in a statement of purpose to say how my point of view has changed since the birth?
No, the statement is about your motivation in doing research and about your research goals, not about your private life goals.

Having a child is undoubtedly a joyful event which can definitely change your perspective on work-life balance, but now that it's changed, simply state what  your current research goals and motivations are.