You need to keep childcount
around longer than just that method. Since you declare it inside the method, it will reset to 0
each time you run this code. Declare the variable outside of the method, so something like this:
int childcount=1;
private void newToolStripMenuItem_Click(object sender, EventArgs e)
{
Form2 childform2 = new Form2();
childform2.MdiParent= this;
string menuname;
menuname = "untilted" + childcount.ToString();
childform2.Text = menuname;
childform2.Show();
childcount++;
}