S., this is very simple when you put the sandbox in,
now it will take a little work to make it, but here is what needs to be done, and you will have NOT ONE weed forever anymore:
UNDER THE SAND< ON THE GROUND,
yo7u need to place a PLASTIC COVER, like a big plastic garbage bags, to cover ALL the soil under the sand.
As you already have sand in, I'd suggest you shovel all the sand from one side to another side of the sandbox,
and then place a plastic film there,
then shovel all the sand on the side where you already have a plastic film, and cover the second side of the soil unbder the sand in the sand box, with the plastic, and even the sand out so that it is everywhere like a sandbox again.
Have fun playing in the sandbox, now !!! :)