I've seen hardcore people wear them regardless of how hot it is. I feel bad for the dude who has to wear a full on jacket and pants who stands in front of the Hogswarts Express. Bless their hearts. I wouldn't recommend it, as it is not only uncomfortable, but in extreme cases you can get sick, or pass out (but I'm sure they would have the sense to take the robes off before that happens).
That being said, if you want to go the comfort route and don't mind bringing something along with you. I suggest that if pictures are really important, that to wear them for certain photo ops and once you're done with photos to have a backpack that has a change of clothes for them to change into.
However, if you mean around right now, the evenings are a little cooler, so you could save the robes for the evenings if you choose to do those photo ops at night.