If they're 3.5 and 20 months are you looking for a big playground or just some swings etc?
On Pewsham there's some swings/slides near Kings Lodge/Tesco, there's a little playground on Wood Lane (between Charter school and where Wood Lane crosses Ave La Fleche) but it's only got a couple of swings and some very low stuff. There's a little set of swings/climbing frame on Blackwellhams off Webbington Road.
Monkton Park's a bit scruffy but it has the trampoline set in the floor which seems quite popular.
There's a nice little park on Stainer Way near Morrisons which is fenced in for little ones.
Haven't been for a while but there's also one near Bumpers with a big slide and climbing frame.
There's a toddler group on Monday and Friday mornings at Station Hill Baptist Church.
We had Bowood membership for a year - personally I think it's very expensive. I get emails every year asking me if I want to renew early for (gasp) a 50p discount off tickets. If you've got children who can use the big pirate ship and you're not too far away so you can drop in often then you'll get value for money but yours are quite young. There is an indoor soft play bit there but it's quite small and very hot in summer. Bowood's only open, I think, March until October?
We now have National Trust membership instead. There's loads of places nearby that have children's activities on and I think it's much better value (and you get to go to different places). Lacock, Dyrham Park, Avebury, Courts Garden etc.