I was in Disneyland at the beginning of July and managed to be there during the hottest day Disneyland has ever had. The temperate hit a record 116 degrees that day and to say that it was hot is an understatement.
That day was an unusually hot day but summer in Disneyland can easily be in the 90s and does get into the low 100s on some days so if you go between May and October you have to be prepared for Disneyland in the heat. That day I was there aside as long as you are prepared Disneyland in the heat can be quite nice.

In California Adventure one of the best places to enjoy the air conditioning is inside the Animation Academy. You can meet Ana & Elsa if you are in the mood for characters, see Turtle Talk with Crush, visit Beast's Library or draw a character. If you do all 4 things it will easily give you an hour in the air conditioning. Another place to enjoy the air conditioning is in the Frozen show. The show isn't quite as good as the Aladdin show that was there before but it is still cute.
Sometimes on a hot day you just need to go somewhere indoors to eat (and have a cocktail). All of the restaurants in Downtown Disney have indoor seating that you can take advantage of but if you walk a little further you get to the Disneyland Hotel which houses one of my favorite spots to have a cocktail. Trader Sam's is not only has an indoor space but despite the fact that it is a bar kids are welcome until 8 PM. The food is good there and the cocktails are even better but the atmosphere is the best.


Final tip for hot weather in Disneyland: DRINK WATER!