Basically for this activity you’re making something that your kids can use as mortar.
Mix wet play sand (we used orange sand as this is what we had) with cornflour in a 2:1 ratioish (I didn’t measure, but about twice as much sand as cornflour, just keep adding sand till it seems right). The mixing is great fun and you find that you go through various stages, all of which are fun to play with. My favourite is when it’s a lot more cornflour than sand and is very gloopy. Once it’s all mixed, if your sand was a bit wet, like mine was, you may find you have a layer of whitish water on top, just pour it off. You then have a very mouldable fun to work with substance, much like moon sand, which you can use to build sandcastles, or as we did as mortar to hold bricks together. Put it in your sandpit, or play bin, add trowels, bricks and toy diggers. Jumbles enjoyed making a tower, though it would have been more satisfying if we’d had play bricks that weren’t interlocking. I tried giving him flat stones, but that didn’t inspire him. Bean wasn’t particularly interested in this activity and took her trowel off to dig in the garden instead.
We also experimented with making bricks using our lego brick mould. This was a bit tricky, they broke apart easily, but was still fun.