Love Cape Town with kids. Most restaurants and vineyards are welcoming and kid friendly.
Some ideas for days out:
Eagle Encounters at Spier Wine Farm
Bugz playpark (best for 3 years + as many rides are restricted for smaller toddlers)
In Cape Town:
Cape Town Science Centre (doesn't look like much from outside but great place to spend a couple of hours)
Aquarium (buy tickets online to skip the queue)
The Blue Train Park and playground at Sea Point
There's a decent playground in the noordhoek farm village
The Franschoek Beer Co has nice outdoor play area and toys
Franschoek Cellar has a covered and supervised soft play
La Petite Ferme has great food and views and a nice green area to play on
Boschendahl had a fab looking new playground under construction last year which is probably open now.
Moreson Bread and Wine has a small play area with toys
Safari Ostrich Farm in Oudtshoorn
Do let me know if you find more as we're going back next year