Don't let the secret out! Many of us regulars on these boards, including me, think July is one of the best months of the year to visit. July Saturdays are particularly amazing crowd-wise. As people have noted, it can get warm especially if there's a heat wave. Heat is also relative compared to what you're used to. I live in Dallas - it is extremely hot and humid in July so SoCal by comparison is tolerable. Even if it's hot it is a "dry" heat compared to Florida or the eastern 2/3 of the country. Plan for daytime temps 85-90, which could be higher or lower depending on the day. It is warmer day-to-day in July, but the most extreme heat in SoCal often occurs in Sept/Oct when there can be days over 100. That can happen in July too, but it's more likely in the early fall.
Go on your dates in July and have a blast! Parks will be open 8-10 (DCA) and 8-Midnight (DL) daily. All nighttime shows daily, max entertainment and max number of rides open. Have a blast!