Pto. Vallarta is fairly safe for families. The hottest month in much of Mexico can be May, because the rain season is normally some time in June through some time in October - but as summer progresses here, it gets progressively hotter and more humid the latter part of the season can see torrential rains, and even the occasional Pacific hurricane. August is a weather crap shoot, IMO. May can be okay because of ocean breezes. September is the hottest and rainiest month.

High season includes Thanksgiving, two weeks around Christmas and the weak leading up to Easter are high peak, and US Spring Break can affect. Prices in Mexico tend to be allowed to rise about mid-December.

Renting a car can be useful, but it really depends on how comfortable you feel driving in an unknown location with a different language, and whether you have plans that will take you up or down coast for excursions or intend to resort sit and hit old Vallarta for shopping, eating, etc. There's a long thread with tips and tricks to renting in Mexico; though some of the rental agencies carry US brands, they are affiliates and operate separately - and of course are bound by Mexican law, not US law.