A safe sandbox is one of the greatest joys of childhood. Weeds in the sandbox are killjoys, so they have to go. Commercial chemical herbicides are great killers -- not only when it comes to weeds, but for other living things as well. They're just not appropriate for use around your precious child's play-land. Use plain old household vinegar to safely destroy the noxious, persistent plants that get in your little one's way.
Pour 1 gallon of white vinegar into a bucket.
Stir in 1 oz. of hand dishwashing liquid. Blend thoroughly.
Funnel the vinegar solution into a repurposed plastic spray bottle.
Spray the solution directly on offending weeds to the point of runoff. Wet all plant surfaces thoroughly. Most, if not all, weeds will die within 24 hours.
Repeat as necessary.