Why did many states have to change their child-labor laws?

Because of mad parents protesting, mainly.

Explanation:

During that time things weren't exactly the safest In the work environment, even the jobs meant for women at the time were much more prone to worker injuries than nowadays.

Now when parents started understanding kids were being injured at these very strenuous/tough jobs they made the government basically enforce child labor laws or else some type of retaliation would've taken place.