Looking at children’s clothing is the most difficult, since we have less representations of them in portraits. Children's clothing followed the same pattern it had for the few centuries before. Generally, boys and girls wore scaled down versions of the clothing that their parents wore. In fact, the children of royalty wore extremely elaborate outfits, as you can see below. Of course, it’s possible that the children had outfits more suitable for play, but unfortunately the majority of paintings from England portray upper class children in their fanciest outfits.
In portraits of families in the middle part of the 17th century, babies and toddlers are often depicted in white gowns. Showing young children like this shows a desire to show comfortable domestic scenes, rather than the formality of the earlier part of the century. By the time children reached age 5 or 6, they would begin to dress like adults. The portrait of the Capel family below , which is styled after a portrait of the Royal family, is a great example of this in practice.
Source: Fashion History Timeline; Fashion Institute of Technology.