Lucerne. It's not far from Zurich but you can take a train where they a family waggon where your child can play while you're talking to you SIL. Once you're there the city is beautiful and the scenery terrific. There is lot of shops so shoe shopping should be possible and as a treat for your child there is the huge museum of transportation (Verkehrshaus Luzern). There is a bus that run from the main station and it takes roughly 10 to 15 minutes to get there. Once you're there you could proably spend hours there

Other than that just stay in Zurich for the shopping and go to the Zoo afterward for the treat. Other possibility would be Rapperswil. If the weather is nice you could even take a boat ride, stroll trough the old town, buy the shoes and go to the Kinderzoo...