As the others posted, July-August can be brutal. The beginning of June isnt as bad but it is still hot. Personally, I would not bring an infant during those months. I remember going to WDW when I was 9 yrs old and my parents standing my 3 yr old sister on top of a garbage can and just continuously pouring cups of water over her head to cool her off because it was so hot. And the people in the park are ruder and less tolerant of one another in the crowds when it is so hot. To me, that isnt a fun vacation.
September-March is much nicer…much more comfortable park weather. Definitely less crowded.