Looking at her she appears to be a TPE doll, so I'll make my suggestions based on that.
This looks like common TPE surface drying and skin fracture at a stress point.
Repair is straight forward. Lightly oil the area and use a temperature controlled hot air gun to gently heat the surface so the TPE melts back together. if you do it gently then there should be no need for extensive surface work to get the skin look back. Search and read a bit before trying it on your doll, and PRACTICE first on a hidden area (neck stump, etc.). Before repair make sure the knee is in a non-stressed position and straight.
Not sure how this was caused, but typically this is the result of leaving a doll in a non neutral position for an extended period. So maybe don't leave her sitting with her knees bent for display. Standing is usually better. Plus drying due to oil leaving the surface of the TPE. She might benefit from a light baby oil hydration to get he back in shape. Some older dolls needed more oil than current TPE blends.
With the oil or heat, take your time, practice first and be gentle.
Go slow if this is your first repair, so you don't make it worse.
Good luck.
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