How to make children look like the reader's customized character

That is a very good question. I think the only way to be able to achieve this is just letting the player either 1.
Customzie the child at the end
2. Ask the player through a multi-choice question about what skin tone they made the parents.
For example:
What is your MC’s skin complextion:
Light
Medium
Dark
What is your LI’s skin complextion:
Light
Medium
Dark

You can then use the power of multiple gains in order to code the skin. The only downside to this is that it might be more complicated to code and that each combination will only have one skin colour .