What do you feed a daddy long legs spider?

The Daddy Long Legs is not a spider but a fly, more correctly known as a crane fly. Adult crane flies feed on nectar or they do not feed at all. Once they become adults, most crane fly species exist as adults only to mate and die.

Unless of course you are referring to the cellar spider which is also known as the daddy long legs spider which traps and eats insects like most other Spiders.