So I've begun re-watching the Marvel movies with my 9-year-old daughter Lia. She's now seen the first Iron Man movie and the Ed Norton Hulk movie. For reference, let's look at the existing films in chronological order.
Phase 1
Iron Man 1
The Incredible Hulk
Iron Man 2
Thor 1
Captain America 1
The Avengers
Phase 2
Iron Man 3
Thor 2
Captain America 2
Guardians of the Galaxy
Avengers 2
Ant-Man
Phase 3
Captain America 3
In my opinion, Civil War and the first Avengers film are tied for best. It's hard to call. Second is Winter Soldier, followed by Guardians of the Galaxy, Age of Ultron, Iron Man 1, Ant-Man, and the first Captain America movie. Iron Man 2 and the first Thor film are probably tied, and then Iron Man 3 and Thor 2 are at the bottom, with the Ed Norton hulk movie taking up the stinker slot.
Lia is already asking tons of questions. Why did the scientist "Mr. Blue" smile when he was (possibly) killed and his face started swelling? How long would the car battery keeping Tony Stark alive last? A lot of her questions were good ones and just pointed out plotholes, but many were answered by "The people who made this movie never let us know because it doesn't really matter."
I love watching the shows I love with the people I care about. It's why I rewatched all of Avatar: The Last Airbender and The Legend of Korra with my family and why we all watch Steven Universe together. Experiencing this stuff vicariously via your kid is just the best.