The claim that the U.S. government and Hillary Clinton "hid and harmed 300,000 undocumented children" is not accurate. While the U.S. government has faced criticism for its treatment of undocumented minors, especially regarding family separations and detention, there is no evidence supporting the specific number of 300,000 children being "hidden and harmed." Hillary Clinton, as Secretary of State, wasn't directly involved in domestic immigration policies, but she has supported immigration reform and protections for undocumented children. This claim appears to be exaggerated or based on misinformation. The source linked with the claim is just a twitter comment, not reliable at all and doesn't have any evidence. As I looked for articles about this topic, I was unable to find anything that was reliable and related to how they were hiding and harming 300,000 children. However I was able to find what her views and stands are on immigration when she ran for president back in 2016, and what she would do to control it. One of her points was, "Immigration enforcement must be humane, targeted, and effective. Hillary will focus resources on detaining and deporting those individuals who pose a violent threat to public safety, and ensure refugees who seek asylum in the U.S. have a fair chance to tell their stories." There are a lot more facts and plans in the article below. Overall this claim was extremely false with no evidence provided.
https://www.hillaryclinton.com/issues/immigration-reform/