17th century: Well-to-do people portray their children
At the end of the 16th and beginning of the 17th century, many well-to-do people had their children painted to show their wealth. See for example the beautiful colf sticks, the lace collars, the rich clothing with beautiful cuffs and rosettes on the shoes.
But also because of the high infant mortality rate, people had their children portrayed. In this way, they were left with a lasting memory.
Even though they appear to be girls, they are really boys. They were often painted at a very young age and then always depicted in dresses.