extension rod and curtain

Something opaque that will let the light shine through. Check out the discount hardware stores. They have thin plastic that goes on bathroom windows to provide privacy yet allows the light to still come through. A bunch of these panels that you put on a painted wood frame would be great.
Go all the way across the room... only leaving enough space for a sliding 'door' at the very end.
The bottom two foot of the divider should be solid wood, (ply wood) that can be decorated however you wish. Then the top part should be hinged and able to pull back when you wish to seperate the room. It should lay flat against the wall when not in use.
This will give an area where the baby can still play with the 9 year old without going into the 9 y/o's side of the room. The bottom of the divider would be a wall or barrier for the baby.

Make sure the part of the wall on the baby's side has a soft lining so there wont be any head bumps or scrapes.

instead of putting your wall unit on the wall use it to seperate one room.decorate it with crystal and plants to give the new section a wider and brighter feel.

I saw a cool one on a TV show. They acquired an old paned window with wooden slates in the window panes. They painted it with fresh white paint. Then they hung it from S hooks in the ceiling with chains attached to the top of the window. The last thing they did was get some of those "fake" leaded glass (stained glass) papers that will stick to glass and put them in all the panes. It looked great, and there was still room underneath of the "window" to put a small table and plant on it.

why not a big peice of lattice it will let in light and also you could decorate it either side with pictures or fabric of either the 9yearolds or a baby choice. Also it can easily be attached and detached from walls or other big pieces of furniture.