Not sure what sort of activities you and your dcs are into (my dc is a bit younger) but here are some ideas to consider:
On a Saturday morning there are two farmers' markets the old biscuit mill is trendy (but lovely) near the city center, and toaki market (over Constantia Nek from Hout Bay) is a bit more low key/family friendly (eg pony rides and jungle gym).
The barnyard has a play area and restaurant as does dunes in Hout Bay.
Kirstenbosch is lovely for walks and you can picnic on the lawns if it's nice weather or splash in the pools if its too hot.
The botanical gardens in town near the museum and planetarium (good for rainy days) have lots of squirrels to feed. There's also an aquarium at the Waterfront or the scratch patch for rainy days.
Sea kayaking (off Hout Bay) or sandboarding if you're more adventurous and the World of Birds (in Hout Bay) if you're not.
The beach at Hout Bay is lovely but (I think) one of the few that allows dogs so watch your step iyswim.
Agree that Boulders and Cape Point are easily do-able in the same day and to try to fit in Chapman's Peak (check first if it's open) and the Constantia wine estates on the way back (if you have time!). Kalk Bay is lovely too (The Brass Bell for a drink/food - right on the water overlooking the harbour).
Stellenbosch: The Joostenberg Deli has a play area and is near to Butterfly World (the name says it all, popular with the younger girls). Second Spier - it's quite commercial but good for cheetas, has horse riding in the vineyards and Moyo is great fun.
If you do want to go further afield past the Winelands, you could try Hermanus (about an hour and a half's drive away) for easy walks in Fernkloof nature reserve and a lovely tidal pool at the Marine Hotel (though it's out of season for the whales there is shark cage diving nearby...).
Sorry if that's a bit long - hope that helps. I'm sure you'll have a great holiday. Enjoy!