If you know people who are not good practising muslims, can you donate money on there behalf? On the day of jugment will these people know that some of there good deeds have come from someone else donating money for them?

Praise be to Allaah. 

Yes,
it is permissible for you to give sadaqah on behalf of your Muslim brother
even if he commits some sins. We ask Allaah to reward you for doing
that. 

There
is no doubt that they will know on the Day of Resurrection that you
have given in charity on their behalf.