Fort Lucan is an outdoor adventure playground which caters for kids from 2-14. We went last summer and the kids had a ball. Friends from overseas have been there too and also loved it.
There is a big outdoor play area which has wooden towers, high walkways, suspension bridges, slides and a maze, which is enough to keep most kids occupied.
Then there is another area, which has two massive waterslides with big bends, twists and drops called "the raging river ride" and there is the "waterfall" which drops you 40 foot down into a pool of water - not for the feint hearted. If you go on these you will get really wet so wear clothes you don't mind getting wet and dirty eg a t shirt, shorts and crocs. Bring a change of clothes!
There are also pedal push go karts and a crazy golf area.
Under 2s are free. Kids cost €9.50 (includes one free go in the Waterfall or Crazy Golf and an adult is €3.50 (incl a free tea or coffee). Individual tickets for the Raging River Ride are €3 PER RAFT (capacity 2 persons per raft and if I remember correctly the raft has to have two people on it to go). Entrance fee does not include this extra cost. If you think your kid will want a load of rides on the waterslides then get a wristband for €6.50 which allows them unlimited access to the waterslides and the crazy golf area. Kids need to be 1m to go on the raging river ride but if smaller, they can be accompanied by an adult.
There is a cafe which sells the usual fare of chicken nuggets, sausages, chips, sandwiches, cookies etc. You can also bring a picnic as there is loads of room to sit on the grass or on the plastic tables and chairs if you can get a free one.