Children are difficult to paint for a number of reasons. They don't sit still, so you might want to work from a photograph if possible. Also, children are proportioned slightly differently from adults. The inclination for some artists will be to make the children look more like tiny adults (case in point: medieval portraits of the baby Jesus). For this reason, some study and observation is necessary before beginning the painting. Painting a portrait is a difficult process and getting a portrait right takes practice and concentration.
Choose your subject. You can work from a live subject or from a photograph. If you wish to work from a live subject, choose a child that is naturally quiet and patient. Have the child sit down and choose a pose that is comfortable. Allow the child to watch a movie or read a book or give the child a quiet and absorbing activity that can be completed while remaining in the same position for a period of time. Divide the sittings into 30 minute blocks (or less) and work quickly. Do whatever you can while the child is away and take photographs of the child during sittings so you will have a point of reference later on. Finally, mark the position of the child with chalk or masking tape for future sittings.
If you choose to work from a photograph, choose a large, high-resolution image.
Determine a medium. Watercolor, acrylic and oil paint are all common mediums used to paint portraits. Acrylic and oil paint look similar when finished, but handle differently during the painting process. Acrylic paint is water based. It dries quickly and can be difficult to work with if adequate blending is important to you. Acrylic is traditionally applied to canvas, but can be painted on nearly anything because it is permanent and flexible once dry.
Oil paint is also permanent, but is very slow drying. During the painting process, it is mixed with turpentine or medium, thus must be used in well ventilated areas. Oil paint can degrade some surfaces over time, so it is usually applied to surfaces that have been primed with acrylic gesso.
Watercolor is dramatically different from acrylic and oil paint. It is somewhat transparent once dry, and is applied to paper in layers. Watercolor is difficult to control and might look very different once the paint has dried. Because it is transparent, it has a luminous quality that appeals to some artists. Artists who work with watercolor must remain open and agreeable to the quirky behavior of the medium, or else expect frustration.
Draw several studies of your subject before beginning your painting. Note that the child's face is probably rounder than an adult's. The eyes will be placed farther down on the head than an adult's. Depending on the age of the child, all the features might be in the lower half of the face, with a wide forehead. The nose will be smaller and more button-like.
Set up the materials and the workspace where it is quiet and free from distraction. You might need to spread a tarp or cloth on the floor. Put on your smock, set out your jars and palette. Situate your subject (if you are working from a live subject).
Draw the portrait of the child on the canvas or paper before beginning the painting. Do not concern yourself with details--only draw the structure of the face and the placement of the features.
Begin painting. Start with large expanses of flat color and build up slowly. Flesh tone is difficult for most people to match. Most light flesh tones are made up varying proportions of red, yellow, white, brown and a little bit of blue or green. Darker flesh tones might use brown, red, blue and yellow or a little white. You might have to experiment with the flesh tones quite a bit to get them right. Note that if you are working with watercolor, you'll want to limit the amount of "experimentation" performed on the painting, because covering up mistakes with watercolor is difficult (or sometimes impossible). However, if you are working with acrylic or oil paint, you can layer your paint freely.
Once you've covered the canvas or paper with at least one layer of paint, you might wish to start adding in details and blocking in areas of light and shadow. If you are working with watercolor, you will never be able to take paint off once it has been applied, so the lighter areas on the painting must be painted lightly from the start, while shadow can be built up over time.
Allow yourself time to step away from the painting. If you are working from a live subject, you will probably already have taken time away from your painting at least once because your subject will probably have gotten squirmy. Make this a stopping point and take several hours or several days away from your piece.
Return to your painting and look for issues. It is almost a guarantee that you will need to make corrections. If you do not know what is wrong with your painting, try turning it upside down. If the features are misaligned or disproportionate, this will likely become evident when the painting is upside down. Try flicking your eyes back and forth between the subject and the painting to look for distortions and areas that need improvement.
Make the finishing touches. Add details such as eyelashes, fingernails and wrinkles in the clothing.
Some features, such as hair and teeth, do not need to be addressed individually, but as a whole. In most cases, you can paint the hair and teeth as one mass, but add hints of individual teeth and hair as needed.