I would just pick up mommy and baby and take them both to a rescue centre, pleading ignorance, they appeared in your garden etc,otherwise mommy is going to be pregnant again and the kitten as well if she is female. People like this make my blood boil!
I know rescues are full to bursting, but there will be a few more soon if this is just left.
Otherwise, ring the RSPCA or CPL for advice, stressing that mommy is only 8 months old now and has the toms after her.
Good luck!