Category menu drop-down not working on one site

I have two sites, and the Category Menu drop-down function doesn't work on one of them. I know in the past you've said this was probably a php problem, but I really can't figure out what's causing it and was hoping somebody else could take a look and give an opinion. Both sites are run on the same server with all the same settings.

Drop-down doesn't work on: http://digitalageparenting.com

Drop-down does work on: http://www.raisedbygnomes.com

Thanks,

Daniel