We paid on the day for our family ticket and that is valid for the year. Weirdly it was cheaper to pay for 2 Adults and 2 child under the family ticket than pay for 2 adults and one child (which was all we needed).
I went 3 months ago for the first time and really enjoyed it (kids will love it too). We found out about MR display when we were there and we will go back in the summer hols to have a look. Th tickets you get are valid for 12 months so you can go again if you want to.