You normally get some form of discount for children, again it depends on the age of the child and the cruise line - best to check their website or phone before booking if this is a worry for you. I know when my daughter was 14months we only paid around £150 for her to sail on the QE2 but in March this year with Island(she'll be 2 1/2) and it's about £35.
No. Prices for children vary depending on time of year. Sometimes they are free but not usually in school holidays. Look on individual cruise company websites.
Depends which cruise company, whether or not you are flying and the age of the children. MSC for example have free kids places on the cruise but there will be a charge fore the airfar.
It depends. Usually the first 2 pax in the room pay the full price - irrespective of age - and then any subsequent passengers pay a lower fare.