I took the original question to be MM asking for music to introduce him to classical music, while being able to keep the attention of a 6 year old, and answered that way. I chose pieces of music which I thought showed the greatness of certain forms, periods and composers, while having great choons (as I believe young people say) and don't need knowledge of the style to allow enjoyment, because of the musical beauty I think they'd be enjoyable for a kid as well. Some of the other answers were great answers to the other reading which would be how do you introduce a 6 year old. MM asked about performances, a purely personal list follows of performances I love of the ones I recommended. I've copied my previous text and added a recommendation in each paragraph.
Hope this is of help.
I think the Mozart suggestion is a good one, Piano Concerto No. 21 as suggested above, or any of Nos 20, 23 and 24, all masterpieces, and very accessible. Murray Perahia's performances with the ENglish Chamber Orchestra are lovely, I'm also very fond of Alfred Brendel's, but I'd start with the Perahia. (For fans, I noticed today that the 12 CD box set of Perahia?ECO is £33 on the river at the moment).)
Romantic period is a good way in, Schubert's String Quintet (D.956) has some of the most beautiful music ever written. (There is also the Trout Quintet which has a piano as the 5th instrument, also a good choice.) Also Chopin, pretty much anything by him, but maybe try a few of the preludes, all for solo piano. For the string quintet, my go to recording is the Alban Berg Quartet with Heinrich Schiff, for the Trout Quintet there is a very good version by the Kodaly Quartet on Naxos, for the Chopin Idil Biret on Naxos is a bargain, or for me Martha Argerich is great in Chopin (not everyone agrees).
For orchestral works Beethoven's best known symphonies, the 6th as a started, then the 9th and 5th, plus a plug for my personal favourite the 7th. Also Mozart, Symphonies 40 & 41 are fantastic. Then look at the Romantic period again, Brahms 1st and 4 Symphonies, Sibelius and Tchaikovsky violin concertos. Beethoven symphonies, I'd go for Nicolaus von Harnoncourt with the Chamber Orchestra of Europe, some people find the tempi too fast, the 1970s Herbert von Karajan are great as well (you can see them easily on a search as the album is on the Galleria label). Mozart Symphony 40/41Harnoncourt/COE again, or Mackerras and the Scottish Chamber Orchestra. Brahms I tend to listen to Szell conductiing the Cleveland Orchestra. Viktoria Mullova's recording of the SIbleius and Tchaikovsky concertos is wonderful, I think her debut album.
Going back to Baroque time, then Vivaldi's 4 Seasons is an obvious suggestion, and I'd also suggest trying Bach's Goldberg Variations, a towering work. 4 Seasons, the Nigel Kennedy recording is excellent, and for the Goldbergs (maybe not a piece for a 6 year old) I'd always listen to Angela Hewitt.
If you like Ma, I guess you like the cello, look out for Jacqueline DuPre's recordigs of the Elgar Cello Concerto, some of the most poignant music ever recorded.
There are so many to choose from, Ive tried to pick ones that don't rely on being steeped in the period's music, great tunes, but still all the genius of the greatest music, which show enough of the forms to help you decide which types you like.
It's hard not to keep listing pieces, I feel I've let the 20th century down, Bach's certainly under-represented, I'd suggest Beethoven string quartets Op. 132 and 135 in a longer list, Mahler and Wanger seem oddly absent, but I'm trying not to just brain dump my whole favourites list.