restart:
with(Maplets):
with(Maplets[Elements]):
M:=Maplet(
 onstartup=A_RunWindows,
 Window[W_Main](
  title="Main",
  layout=BoxLayout(
   BoxColumn(
    BoxRow(
     BoxCell(
      Button[B_Child_1](
       caption="Child 1",onclick=RunWindow(W_Child_1)
      )
     ),
     BoxCell(
      Button[B_Exit](
       caption="Exit",onclick=Shutdown()
      )
     ),
     BoxCell(
      Button[B_Child_2](
       caption="Child 2",onclick=RunWindow(W_Child_2)
      )
     )
    )
   )
  )
 ),
 Window[W_Child_1](
  title="Child_1",height=100,width=200,xcoord=100,
  layout=BoxLayout(
   background="#AABBCC",
   BoxColumn(
    background="#AABBCC",
    BoxRow(
     border=true,
     BoxCell(
      Button[B_Close_1](
       caption="Close", onclick=CloseWindow(W_Child_1)
      )
     )
    )
   )
  ) 
 ),
 Window[W_Child_2](
  title="Child_2",height=100,width=200,xcoord=700,
  layout=BoxLayout(
   background=COLOR(RGB,0.9,0.5,0.4),
   BoxColumn(
    background=COLOR(RGB,0.9,0.5,0.4),
    BoxRow(
     border=true,
     BoxCell(
      Button[B_Close_2](
       caption="Close", onclick=CloseWindow(W_Child_2)
      )
     )
    )
   )
  ) 
 ),
 Action[A_RunWindows](RunWindow(W_Main))
):
Display(M);