I tried to read all the posts, so I hope I'm not repeating something.

If u want to get those kids out keep lots of tape for that cam corder around & keep it pluged in with an extention cord..
Everytime u see her yelling or harming the children, record it. If the little girl is outside, take the came corder over to her & ask her whats wrong & where r ur parents & so on..

Thats hard evidence. Just remember to keep the date & time stamp up to date & mark each tape for day & time.

Also if u can, make copies of the tapes. I don't trust anyone & it would be terrible if the tapes went missing or broke.

If she says anything to u, record it & call the cops right away. Give them ur tape & show them the other tapes. Cops don't like child abuse & should help u out by keeping evidence & a record of all the terrible stuff going on over there.