If you MUST have a V8, then you'll have to book through Enterprise Exotics to guarantee a V8 (they call the class "American Muscle").

You can rarely find a Mustang GT or Challenger R/T (5.7 not the 6.4) on the lot at Enterprise/Avis/etc. I would not bet on this though.

Comparing Mustang/Camaro/Challenger, the Challenger has the most room in the backseats. Doable for road-trips if your kids are small (5'8 or less I'd say to sit comfortably in the back long-term). Camaro and Mustang not a realistic choice for more than 2 people (driver + passenger). Also consider Dodge Charger (the sedan) and Durango R/T (V8 SUV).

Your best bet may be Turo. If specific car engine is very important to you, I'd recommend that because you know exactly what you're getting.