This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
Lighting. As previously stated, supply all the light possible, and while ordinarily a room with a northern exposure would be preferred, yet where the sunlight falls upon the window, with its angle in the direction of the space to be occupied by the subject, you will have stronger illumination, and if the sunlight does not come within the picture space it will do no harm. The light will be very much diffused by tacking a sheet of white muslin over the window, and the subject may be placed quite near without any harsh effect.
Plates To Use. None but the most rapid plates should be employed for children's portraits, enabling you to make reasonably quick exposures and obtain expressions which could not be secured if slow plates, requiring long exposures, were employed.
Exposure. The required exposure will depend upon the strength of illumination, also the speed of the lens employed. Use the lens at its largest aperture, thus allowing for the quickest possible exposure, which will vary from one-half second to two seconds, depending on the light and the speed of the lens.
Development. Use any developer. The Universal Developing Formula given in Volume II is recommended, but as exposures made under such conditions are very apt to be short, double the amount of water should be added to the regular formula. This will give you more detail and a softer negative.