The Piano Bar in Ambleside has a soft play area in it (I think!) - otherwise you will struggle for soft play places in The Lakes I think (there is one in Barrow but it's a bit of a drive from where you'll be). Ambleside is a nice place to potter round in itself. There is a big garden centre there called Hayes - ask anyone for directions because everyone knows it. I think there is a play area outside there and it's a massive garden centre with nice gifty type shops in and also a cafe - nice for a potter round if it's wet.
There is Aquarium of the Lakes at the side of Windermere - quite expensive though I think. From here you can take a ride on a little steam railaway - it's the Lakeside and Haverthwaite railway. That's quite sweet and not expensive. At the other end of the line from the aquarium there is a tea room and lots of old engines in teh yard if your dc is into that?
Also from lots of points on Windermere you can get a boat trip.
Fell Foot Park is a NT place on the otherside of Windermere - it's lovely there - nice play area, little cafe, nice place to walk round. From there you can get a very short boat ride over to the other side where the aquarium is - it's only £1 or something.
Bowness-on-Windermere is a really nice place to go around as well - again, you can get a boat trip from here (e.g. to Ambleside or back to where the aquarium is). I think you can do an open top bus ride here and there is also the Beatrix Potter place (although it's not cheap).
If you fancy doing a walk and you've got a car to get to it - Tarn Hows is a nice walk to do with children. At one end of it there is a little stream that is really shallow and lots of children potter about in there with their wellies or crocs on. There are no toilets there (but lots of trees!) and there is usually an ice cream van in the car park.
Will that do to start off with?!