Click here http://www.rsdb.org/ for a definition of linthead. (It's a racial slur).
'younguns' refers to young children, and 'they' refers to mothers:
2. With three or four young children, mothers are held down the same as if they had on chains.
I'm not sure what 3. means. Sorry.
3. I sacrificed the things in hand for the good of the hypothetical whole.