When my daughter was about two months old I used a large mirror placed on the floor and put her on it on her tummy and arranged her hands in a way that looked natural. I then tried different angles shooting from over her head where I could see her reflection. This could work for a 4 month old as well.
As for an 18 month old, maybe give him some cute wooden toys or classic children's toys and let him play with them. Call his name or have his mother get his attention when you want him looking at the camera.
at 11:52 PM on Mar. 7, 2009