How can I make an adult's face look like a child's? Or where can I learn?

And I recommend you apply pretty much all Filters you use on the image itself as Smart Filters to avoid unnecessary image degradation.

Good day!

Whether you can do it in Photoshop depends on your skill and the amount of time and energy you are willing to invest, I guess.

So far Photoshop has no »make young«-Filter but several existing features can be used to emulate various components of the intended appearance: certain Filters may help in reducing skin blemishes and wrinkles but manual painting may still be necessary; as for addressing shape and proportions Liquify is a good choice but you need to make the decisions so you ideally should have some understanding of how ageing affects anatomical features in humans.

Regards,

Pfaffenbichler