Whether or not "Kids are Welcome," depends on you and how you are planning your wedding. Do you want to have an "Adults only" wedding?
As far as the RSVP deadline, I think that usually depends on when you need to have a final head count to give to your venue/caterer. Then, work back from there. If they need to have a final head count 2 weeks prior to the event, then I would say that the RSVP deadline would be 1 month prior to the event. If you know that you have a lot of people in your guest list that are "procrastinators", then give yourself that amount of time as advanced notice.
Re: TWO QUESTIONS
As far as the RSVP deadline, I think that usually depends on when you need to have a final head count to give to your venue/caterer. Then, work back from there. If they need to have a final head count 2 weeks prior to the event, then I would say that the RSVP deadline would be 1 month prior to the event. If you know that you have a lot of people in your guest list that are "procrastinators", then give yourself that amount of time as advanced notice.