There is only one PROMO video from 1979 (no.1 below), but as there are different versions of I Have A Dream, they are all listed here for clarification:

1. 1979 PROMO
   Info from the booklet from CD/DVD Voulez-Vous Deluxe edition: In conjunction with ABBA's tour of Europe in 1979, their November concerts at London's Wembley Arena were filmed for the television special ABBA in Concert. Before the special had been put together, however, I Have A Dream was rush-released as a Christmas single. Wembley footage of the song was used as a promo clip, which can be viewed on the ABBA Gold DVD. It can also be seen on the video from 1989: A Video Biography 1974 – 1982.
   
   
2. Extended PROMO
   Info from the booklet from CD/DVD Voulez-Vous Deluxe edition: There was also a version of the clip featuring an encore, which has only rarely been seen since ABBA's heyday and was released on the CD/DVD Voulez-Vous Deluxe edition (released May 2010) for the first time. The difference between (1) above is that at the end of the song, Björn says, "Great kids! Kids here from London. Do you wanna hear that again? OK!  I told you so! One more time!"  There is then a encore where you see Tomas Ledin clapping with his arms outstretched and Agnetha is leaning over a dark-haired girl. Björn then says, "Thank you. Thank you for coming, you were really great." The reprise part only was included on ABBA: The Golden Years (basically The Story of ABBA from 1982).
   
   
3. ABBA in Concert
   The version included on ABBA in Concert and released on the DVD, (included here just to highlight the differences) is largely the same as (1) and (2) above but sometimes shows different (and more) camera angles of exactly the same performance - while other shots are completely different.  The main difference is in the instrumental break. This version has the choir rehearsal footage (with Björn looking worried and at one choir and pleased with another ("You're one of the best choirs we've had so far").  At the end, Björn says, "Thank you. Thank you for coming, you were really great," (as per (2)) and adds, "To remind us of UNICEF and the year of the child."

You could try to view all three videos together at the same time but you need to be careful with your start points and pauses to get them to all play in sync and therefore see where the differences are.

Release information:

1. The 1979 PROMO was released on the ABBA Gold DVD.
2. The Extended PROMO was released on the CD/DVD Voulez-Vous Deluxe edition in May 2010. 
   The reprise part only was included on ABBA: The Golden Years (basically The Story of ABBA).
3. ABBA in Concert DVD

The children you see in close-up are not the same ones that are in the distance line-up proving that the choir footage was mix-and-matched.  The following kids cannot be seen in the choir shot:

1. Linda
2. The snotty kid!
3. The girl who looked round at Frida and Frida touched her face
4. The boy in the checked shirt
5. This girl

However, the dark-haired girl seen with Agnetha in the encore (2 above) IS in the line-up - 4-5 from the right-hand side.
 

NOTE: The images above are not from the Voulez-Vous Deluxe edition, but from a 16mm print and are more colourful than the official version. These captures were sent to me, I do not have a copy of the print.  Also, the images for (3) are not included here but in the entry for ABBA in Concert.

++++++++++++++++++++++++++++++++++++++++++

One of the versions is particularly annoying as you hear the introduction to Does Your Mother Know from the ABBA Live album right at the end, when in fact there was an encore of I Have A Dream at each concert.  Which version has this?

Björn and Agnetha's daughter Linda in the children's choir.

Lyrics available on the Official ABBA site

ABBA Live in Copenhagen, 1979;
ABBA Live in Dortmund, 1979;
ABBA Live in Bremen, 1979;
ABBA In Concert, 1979;
I Have A Dream PROMO, 1979;

Thanks to Matti Crocker, Chris Williams, Alex Jones & Gabriel Zubowski via ABBAMAIL (no longer online). Extra thanks to Jozsef and Chris Parker.
YouTube clips c/o NikeDye, Patrick Remes and 7ABBA7.