If water remains in the sandbox, the problem might be that the sandbox does not discharge the water effectively. This might be related to the substrate. In any case, we recommend not placing the sandbox in the middle of a 100x100 concrete tile or on a terrace with cement joints.
In addition, the cover of the sandbox does not keep the sand dry. Actually, our covers are not watertight, which keeps the sandbox sufficiently ventilated and prevents the formation of mould.