Somerset West will mostly involve a bit of driving if you want to head back to Cape Town, but it's not that far.
Strand beach is near Somerset West and is a lovely long sandy beach. I haven't been there in a long time, but I imagine you'll need to look out for lifeguard sections for swimming.
If you're going to the Aquarium at the Waterfront, plan to make a full day of it. The waterfront is great fun with kids and there are play areas etc too. Save a an hour or two for the Scratch Patch - there's one in the waterfront and basically, the kids get to scratch around for "jewels". Honestly, it is the most fun you can have with any child under the age of about 10. The one in Simonstown is bigger, but further away so I wouldn't bother https://www.scratchpatch.co.za/scratch-patch/ The Aquarium also has a local Build A Bear that is a LOT cheaper than in the UK so if that's something you've ever wanted to do.... take the gap.
Check out what's on at Spier wine estate. They do various family events and I see they have some Easter activities if you are going in the upcoming holidays. https://www.spier.co.za/events. It's also conveniently located for Somerset West.
There are a couple of trampoline parks. What's great about them in SA is that there will be staff who will actively play with your children so you don't have to! 
. I consider this a win.
Other beaches that are fun for this age are Fish Hoek, muizenberg, St James. To be honest, our children just generally love the waterfront and the beaches so it's hard to drag them away from either.
Near Seapoint, there's a lovely play area and a train park https://thebluetrainpark.co.za/
A note that Spur restaurants are very family friendly and will often have soft plays, face painting, arcade games or even entertainers for the children. Also, South African milkshakes are better than England's so definitely prioritise getting some .