Children is Stardew Valley is a nice, though very basic feature.
You can only have two of them. One male, one female. Gender of the first child is randomly picked, gender of the second one will always be the opposite.
There are two skin tones for children: white or brown. I believe they are unrelated to farmer's skin tone (especially as the player can change it for the farmer, which isn't possible for children).
Children will never grow past toddler stage. Interaction with them is limited: they don't talk, don't accept gifts, never leave the house except during festivals, never give anything to the farmer.
They have no other customisation option but for wearing a hat, which still don't make them half as useful as a rare scarecrow or a horse. We can compare children to urchins living outside a fish tank, though. That's still a thing.
Thus, I agree with you that skin tone for children could be improved, but I belive that the whole children feature could take many other improvements also.