Cute little railway run by enthusiasts with nice old station building and quaint cafe. The line runs from the station to Newmills where there is a playfarm. This is basic to say the least (sand pit, swings, ride on toys, farm animals to see) but does keep the kids entertained while you wait for the next train back.
Lappa Valley Steam Railway (not far away) is probably a better bet though - has three different little railways and nicer play areas with boating lake etc...