If it is Gullivers World in MK and its a nice day then literally across the road is Willen Lake
A lake, obv....take a loaf or two of bread to feed the many different birds/ducks/geese etc
Nice sized kids park alongside too
somwhere to eat and drink too,
a maze
a kids train ride
and if they are adventurous the small kids part of
http://www.aerialextreme.co.uk/page/milton-keynes is by the lake too