The 1st Floor restroom in the Rose Center/Hayden Planetarium is in the southwest corner, behind the elevators.
On a side note, the signage in that museum is maddening, ridiculous. Veteran visitors thread their way through lost, baffled, shuffling newbies.
My suggestion: get a membership as soon as actually useful signs and more than one hyper-congested stroller entrance are in place. Until then, "suggested entrance fee" means $1/head for us. [Believe me, though, they'll still make it up in volume.]