There are a few places which sell Clarks shoes around. The Suntec shop has a limited selection of kids shoes, the majority are adults. I think there is a Clarks shop in UE square or United Square (are they the same place?).
Kickers shoes might also be suitable. I know they do some black and brown girls shoes - don't know about boys as I haven't got boys.
You can buy them from a shop called Pretty Feet (or Fit?) Level 2 Suntec and also in Centrepoint - I can't remember the name of the shop there, but seem to remember it being on level 2 on the side where the Coffee Bean is (almost above)
hope this helps