The first and original release is better to start off with IMO.
Second is still fun, has a different feel to it and the split screen works well for two players. Buy this one later though. Also try the Lego Star Wars Complete Saga if you havn't already. It's very cheap now and a classic! Lego Batman is ok but wait till the aforementioned others have been played first.
I would recommend the 1st Lego: Indiana Jones for that age group. My 2 boys (ages 6 & 8) love to play it together. We rented the newer one, and although they liked it, I thought the controls were harder. When Indy uses his whip, you need to aim your bullseye on the screen, while in the older one, your character just had to be pointed in the right direction. Took me a while to catch on, though the kids got it soon enough. I think that the games are different enough that I might end up getting both, but since the older one was only $20, it's hard to beat. It also seemed that in order to cover 4 movies, there's less depth in each one now.
And I agree that Lego:Star Wars (for same price) is another winner. There's lot of game play in both of these, and my kids keep playing their favorite levels over and over.