90 degrees is not hot enough to be a danger though they need shade and clean water. Adding extra heat is not a good idea though.It’s currently 90 degreees where I live, so I’m thinking they’ll be ok,
If you were concerned about cold temperatures then the concern would be how cold it gets, not the warmest. For what it's worth I've had chicks go through temperatures in the mid 20's Fahrenheit when they were younger than yours.
Seriously, I'd get them out now so they can acclimate to the heat. That is going to become more of a concern than cold. Make sure they have shade, water, and good ventilation.