Seriously, I am unable to really find much about them outside of short lines from Marx and Mao about their potential destructiveness among other things, but I still do not really know what that “class” is. It seems to refer to the poorest members of society that includes unemployed, criminals, homeless, etc… And are they really so incapable of being utilized in revolutionary activities as they are portrayed?
Edit: By “destructiveness”, I refer to how Marx and Mao portrayed them as people that are not considered reliable allies in any proletarian revolution (though even this understanding might be wrong because I think the explanations about them are vague).
I think that class can be determined at the family level in the case of children, who if raised by proletarian parents are entirely dependent on the wage labor of their parents to live, so I think considering them a part of the proletariat is correct in that circumstance. However, if you are an adult separated from your family in some way and are capable of selling your wage labor (if you could not, then you would be like the children too young to work for a wage and your class would depend on those willing to support you), you yourself determine the class you are a part of.
It might also be possible if they developed class consciousness as a proletarian before becoming a lumpenproletarian, but that is a specific circumstance that not all lumpenproletarians will have (I happen to be a proletarian that might go homeless soon due to personal choice aggravated by difficult home circumstances). I also want to add that the lumpenproletariat (even those who are Marxist) are difficult to use because they have a hard time even surviving, so they will be focused on that rather than protesting or leading a revolutionary movement against American capitalism; in order to utilize them, you must uplift them out of their dire straits through financial means (alongside other aspects).