Actually, I have a large sandbox in my backyard(on fake grass carpet on top of blacktop). You CAN wash the sand in your sandbox, and you CAN sanitize it. Two ways, both involve bleach in some form. Regular Household or Medical Bleach or Pool Chlorine. Judge how much you need depending on the size of your sandbox. If using household or medical bleach, dilute 1:1. Pool Chlorine, disolve any powder well, and use your judgement on how much to use per gallon of water. Use more than you would for a pool. You can use hot water if it makes you feel like you're doing more.
FIRST, you MUST sift through the sand to make sure you got all little presents. If you have a bottom on your sandbox, hard or cloth(like mine) turn all the sand from one side up and pile it on the other side so you have the bottom exposed. Spray or pour some chlorine/bleach water there, put the sand back in layers and spray/pour more chlorine/bleach water over each layer. Do the same on the other side. Then COVER the sandbox. Sunlight destroys chlorine(and chlorine is in bleach) so it needs to be covered for a while to allow disinfection. Judge how long you think is necessary to keep covered. I do this overnight. Then uncover and let the sun dry out the sand. You could rinse the sand with the garden hose before you let it dry if it makes you feel better. I don't. I just don't let the kids in until I feel it's ready, nor do I let them wear clothes I care about in it.
Rake the sand to make sure you get all the sand dried out. This may take several days depending on how big your sand box is. You can do this as bleaching process often as you think necessary. Word to the wise, wear clothes you don't care about or white while doing this. Pool Chlorine bleaches clothes too. Always cover the sandbox at night or whenever not in use to keep the critters out. Use cinnamon powder in the sandbox to keep the bugs out.