This seems a great deal to me for a fun day out at a heavily reduced price, all you have to do is book a ticket 3 days or more in advance (which i always do anyway to make sure it's available) throughout march and you'll get 55% off adult prices, making it only £14 per adult instead of the standard £31 and child tickets are the same price.
If you can't make it for March, they also have the 3 days advance booking offer throughout April at a slightly higher price, making it £18.50 per adult.
Don't forget to go through Quidco to receive 3% cashback.
Hope this helps and you all have a great time throughout the easter hols. :)