I don't really care about US politics, but I find data quite interesting. Now I would not say this is outright false, but highly misleading. A quick search led me to this csv of to whom minors were released: https://raw.githubusercontent.com/nytimes/hhs-child-migrant-data/main/data.csv Running a quick script on my phone, 41.7% of the 553321 children were released to a parent, and further 46.7% to a blood relative (brother/sister, aunt/uncle, grandma/pa). So only 64340 were in the other category, which includes family friends, other distant relatives and unrelated sponsors.
Now I will not go into if 64 thousand children is "good enough", or doubt that many of them work. But it is not a problem exclusive to unacompanied minors. I would think that if parents force their children to work, that is regardless of if they came alone or with their parents.
I would say the tweet is highly missleading, because it makes it seem as if there are 400 thousand abandoned migrant children working in the US.
An other fact to consider, is that in 1860 the US population was 34 millions while it currently sits at 335 million. So comparassions of absolute numbers in a historical context are usually not a good idea.