There actually is a reasonable explanation for some of this. Many aristocrat children were dress up by servants to look exactly like the adults. Wigs, suits, make up, you name it they used it. That being said, the ones in the painting are just ridiculous. They are not proportioned right to be children. My guess is that they are not actually representing children but some kind of artistic representation of little man syndrome or something similar.
or maybe he was just running out of paint. but he wanted to get this done but didn't want to go all the way to the paint store, it was like Friday evening and traffic and all.
Like adults but smaller.
Got it!