The dimensions are important because cichlids for the most part like length in which to establish a territory. I assumed that it was four feet long but its always better to be sure.
If you did only yellow labs in the tank a good number would be about 10. I think that only one type of fish would be dissapointing for young children though. The benefit of the labs is that they are very peaceful as far as malawi mbuna cichlids go. A downside is that these fish are often difficult to find as a pure bred species because they are often hybrids with the more aggressive Metriaclima estherae (usually sold as red zebra).
Bright colorful fish that can work with yellow labs include peacock cichlids. I don't have any experience with peacocks though I know the females are usually drab in coloration and that if kept with too aggressive of fish the males don't color up properly due to stress.
Another set smaller mbuna include the cynotilapia afra variants, many which are stunning like the jalo reef:
http://www.cichlid-forum.com/profiles/s ... hp?id=1733
I hate to push you in any one direction since there are over 1,000 species of cichlids but I think that mbuna are a great choice for children since they are so colorful and active. There are a lot of species that can work with your tank.
I just got an idea, you could do a psuedotropheus saulosi tank they look like:
http://www.cichlid-forum.com/profiles/species.php?id=1
females are a nice mango color and the males color up with light and dark blue stripes. There is someone on the forum who's name I forget that has a really nice all saulosi set up. Sometimes only the dominant male will show the best colors and its important to keep a ratio several females per each male. This can be difficult to achieve though because juveniles all have female coloration. Sorry to innundate you with information about fish that I happen to like. To make up for it here is a fish I personally don't love that I think that could work, the rusty cichlid:
http://www.cichlid-forum.com/profiles/s ... php?id=707
Wish I could be of more help but thats all I can really think of at the moment. There are others on the forum though who are basically encyclopedias of fish information that would be able to give more options.