I specify canon because there is tons of fun AU and just unofficial Star Wars stuff that is fun for kids, like the Droids animated series, the Ewok adventures, the 2003 Clone Wars series (made by the Samurai Jack studio) and the Lego Star Wars films like "The Empire Strikes Out."
1: The Clone Wars (2008) - This is seriously great. By the time you are halfway into the first season, you will be wishing to god that the prequels had been made by these people instead of Lucas. They are exactly what we all hoped the prequels would be - the awesome adventures of Obi-wan and Anakin. You will find yourself caring about both of them, along with Padme, Count Dooku and several new characters like Ashoka and Asajj Ventress. Even Jar Jar is retconned to be somewhat less offensive. Your kids will love this show and be excited by it, and so will you. They will go into the films knowing about Anakin, Obi-Wan, R2D2, C3PO, Padme and the Clones, and it will only enhance their experience.
2: The Phantom Menace - This is a god-awful racist heap of shit, but watching Clone Wars will make you totally want to go back and see the origins, botched as it indeed was. And it ends with one of the greatest lightsaber duels of all time, so you'll at least have a high note to finish on (and a thing to keep you watching in order to get to). Your kids will probably not hate it (though they might be bored by it at times, as it is far longer than any film that boring has any right to be - If any film can be chopped up and watched 40 minutes at a time, it is this tedious piece of shit). That is ok. They are excited to finally be seeing these characters come to life, and are too young to understand how awful it is. Your job is just to grit your teeth and stay strong until the lightsaber duel.
3: Attack of the Clones - Once again, mostly shit, but your kids will be excited to finally see the Anakin and Obi-wan they remember from the Clone Wars (seriously, that beard makes a huge difference, Ewan), and now they will be less disturbed by Anakin/Padme. Though perhaps they shouldn't be.
4: A New Hope - Unless you are a terrible fucking parent who has isolated your child like a crazy Star Wars fundamentalist, your child has heard of Darth Vader. If you're lucky and careful, they might not have heard "Luke, I am you father" (they certainly haven't heard it from the films, since that line isn't actually from any of them). Either way, most kids accept it at face value when Obi-wan tells Luke that Vader killed his father. They trust Obi-wan, and they will be super sad about Anakin's death, but will totally accept it, and the awesomeness of Luke and Han, combined with the familiarity of R2 and C3PO, plus finally seeing Tarkin (who you constantly want to slap in The Clone Wars) get blown to smithereens will get their spirits up quickly enough.
5: The Empire Strikes Back - Obviously they will love this one, because it is the best. Plus, they will literally have the same reaction as Luke at Vader's Cloud City revelation. Like, they will simply not believe it at all.
6: Revenge of the Sith - Here is where you watch their little hearts break forever
7: Return of the Jedi - And then you all get to see Anakin finally redeem himself! Does he bring balance to the force? Eh, doesn't really seem like it. But still, I think that the prequels (and especially the Clone Wars) actually make RotJ a much more satisfying conclusion.
MINOR SPOILERS BELOW AND BIG ONES IF YOU CLICK THAT LINK
8: The Force Awakens - Obviously, this comes after. And also, Rey is totally a clone of Padme.
I haven't seen Rebels yet, so I will update this list after I watch that, and again as Disney inevitably wreaks havok on continuity with a Boba Fett film and a Han Solo film.
