A quick search on the topic of child labour and its impact on the health of the worker led me to these two papers: The Health Impact of Child Labor in Developing Countries: Evidence From Cross-Country Data and Environmental and Occupational Health Problems of Child Labour: Some Issues and Challenges for Future.
They focus mainly on labour in hazardous conditions, which may not be your central paradigm (although I doubt Cinderella's adoptive family cared much about her health and safety), but they also list a great number of criteria that make children more vulnerable than adults, such as:
- Greater toxin absorption, both through the skin and lungs
- More susceptible to toxins, due to an incomplete growth, especially in the brain
- Greater energy consumption, so they will suffer more from malnutrition
- Greater sensitivity to heat and cold, which will increase their fatigue both short and long term
- Weaker immune system, in part sue to all of the factors listed above but also just naturally
All of these mean that any effects of intense labour you'd observe in an adult will be exarcebated for an adult who had to endure these hardships as a child. Feel free to pick from this list of effects you could see:
- Scoliosis, or other back problems, that can lead to someone being hunchbacked, after spending years scrubbing the floor
- Reddened eyes and skin, in sensitive areas, due to exposure to toxins
- Breathing problems, such as shortness of breath, asthma, bloody coughs etc, again due to toxins
- Scars due to infection, particularly on the face (puberty is kind to none...)
- Perhaps dormant infections, contracted young and just waiting to break out in later years
- Calloused hands and feet, and probably damaged nails (ingrown, infected)
- Unbalanced strength, because Cinderella would probably prefer using her dominant arm for scrubbing
- Tendinitis and arthritis due to repetitive movement (these can manifest later on, a 20 year old with the tendons of a 35 y/o wouldn't necessarily have tendinitis, but a 40 y/o with the tendons of a 55 y/o would)
- Stumped growth and weak bones due to malnourishment
- Crooked and damaged teeth
I mentioned that a fair amount of these can manifest later on in life because one concern of mine is... Would Prince Charming really fall in love with a female Hunchback of Notredame? I know beauty is in the eye of the beholder, but still. It would probably make more sense for Cinderella to be slightly bashed from her labour, then Fairy Godmother undoes some of it temporarily for the ball, only to have it come back at midnight (hence why Cinderella ran off). Then throughout her later years, she develops other complications due to her messed up childhood.
Another factor to keep into account is that children are very vulnerable psychologically too. Years of abuse and neglect will leave her damaged in many many ways. This could be another point to exploit in her bitterness later on ("Step-Mother never let me eat enough as a child, so I'll be sure to have a massive feast every night now that I can"). Her various aches and pains (you could probably even call them disabilities if they've advanced far enough) probably wouldn't help with this aspect either.