Tough, tough call! Seeing as it's a family trip, my first choice is just the 24-105. But to answer the original question regarding primes: 24mm and 50mm will do what you need. It'll be challenging using just two focal lengths, but that was the pair I carried throughout Italy a few years ago, and it was a ton of fun! But that was then....
Now, my travel set up is the 16-35mm f/4 and my 50L. Maybe throw the 100L in if expecting cool bugs or fauna.
I say purchase the 16-35 f/4.



