Went last week to the same area with 18 month old ds. Fab holiday.
Again really recommend steam train from Corfe to Swanage. You can park easily at both ends - the Corfe end has a 'park and ride' area called Norden (I don't think it's the same as Corfe station).
The model village in Corfe quite sweet, and they have some outdoor games in the grounds (over sized chess etc) - obviously not playing but causing havoc at 18 months, but he loved it.
Putlake adventure farm in Langton Metravers was brilliant - adults allowed in soft play area (wearing socks!) - I'm usually not a fan of soft play things but this was a really good one. All very clean and family run. Lots of lamb and goat feeding, little tractors, crazy golf, sand pits etc. Was very quiet when we went.
Pub called Scotts Arms in Kingston has a few little tykes type slides etc in nice garden with lovely views, and food pretty good too.
Kingston Lacey is a NT house with huge grounds and a picnic spot (nice but near the car park) and three playgrounds (although didn't get to them). If you are keen on going into the house there are toys for the children to play with in the attic room. Lovely big lawns and a great cream tea.
Hope you have a great time.