Some babies sleep better in a very dark room. The following are some ideas for achieving this.
Tin foil taped to the inside of the window works very well. It may look a bit odd to the neighbors, but they will get over it.
And probably copy the idea for their own kids.
Tagboard or cardboard can do the same thing. A bonus: Have your older kids decorate both sides with crayons/markers. Or have a colouring party with your spouse.
You can buy special blackout shades and curtains.
Hanging blankets or towels over the curtain rod, in front of the curtains. Watch out for too much weight. You don't want a middle of the night crash scaring the daylights out of everyone.
Buy a twin or double flat sheet. Cut it in half lengthwise and fasten to the inside of the curtains with safety pins.
Our dear administrator Gareth suggests bricking in the window with cinder blocks.
Can we say "warped sense of humor?"
G!
Tape or velcro the edges of the curtains to the window frame.