re: Keeping ants out of a sandbox?

Just price some Bifenthrin (whatever version of generic Talstar your local hardware store has) and spread that. 6-8 months of ant free yard. I treat over 30 acres of kids' baseball fields with it every year. It is safe, and extremely effective.

ETA: if you can't find any, let me know what size area you want to treat and I will price it for you through my supplier. I am ordering soon and will get a bulk discount.