One lens that I have for a while and love using on full frame is the Nikon 50mm f1.2. Actual performance at f1.2 is not that great, with very high light fall off and some overall softness, but the images really do pop with the shallow depth of field when you get it right.
However shooting a toddler near minimum focus distance at f1.2 with manual focus on a full frame camera is close to impossible. You only have less than 1cm available to you to nail focus, and as they move so quickly you don't get too many keepers. Better to use it as a night street shooter, like this shot of Nuit Blanche in 2010 on the D700.