My fiance is going to get visitation rights to his kid on the weekends. Can I legally go with him to his exes house to pick up his kid?
Yes, you can legally go with him.
If you walk up to the front door with him, and the property owners (or the persons RENTING the property) ask you to leave, you probably should.
If you're walking up to the front door of an apartment, you probably have the right to stand there as long as you want to.