private void Form1_SizeChanged(object sender, EventArgs e)
        {
            //Form2 subform = new Form2();
            if (this.WindowState == FormWindowState.Maximized)
            {
                foreach (Form obj in this.MdiChildren)
                {
                    obj.WindowState = FormWindowState.Maximized;
                    obj.Show();
                }
             }
         }
实现了，测过

------解决方案--------------------
C# code

        private void main_SizeChanged(object sender, EventArgs e)
        {
            foreach (Form obj in this.MdiChildren)
            {
                obj.WindowState = FormWindowState.Minimized;
                obj.Show();
            }
            if (this.WindowState == FormWindowState.Minimized)
            {
                this.cmsNinSystemShow.Enabled = true;
                this.cmsNinSystemHide.Enabled = false;
                this.ShowInTaskbar = false;
            }
            else
            {
                this.cmsNinSystemShow.Enabled = false;
                this.cmsNinSystemHide.Enabled = true;
                this.ShowInTaskbar = true;
            }
        }

------解决方案--------------------

C# code


        bool isno = false;//控制是否第一次改变。因为第一次打开主窗体里就会执行Resize事件
        int IntoHeight, IntoWidth;//放入每次改变时的父窗体值
        float TempHeight, TempWidth;//获得每次放大放小的比例值

        private void main_Load(object sender, EventArgs e)
        {
            isno = true;
            IntoHeight = this.Height; IntoWidth = this.Width;
        }

        private void main_Resize(object sender, EventArgs e)
        {
            if (isno)
            {
                TempHeight = float.Parse(this.Height.ToString()) / float.Parse(this.IntoHeight.ToString());
                TempWidth = float.Parse(this.Width.ToString()) / float.Parse(this.IntoWidth.ToString());
                IntoHeight = this.Height; IntoWidth = this.Width;
                ChangAllChildRenSize();
            }
        }

        public void ChangAllChildRenSize()
        {
            foreach (Form tempChildForm in this.MdiChildren)
            {
                tempChildForm.Height = int.Parse(Convert.ToString(Math.Round(tempChildForm.Height * TempHeight,0)));
                tempChildForm.Width = int.Parse(Convert.ToString(Math.Round(tempChildForm.Width * TempWidth,0)));
            }
        }