Como en Casa makes DElicious cakes. Nucha makes great cakes as well. However both of these are pretty adult places, though Nucha may do kids birthday cakes as well, or possibly cupcakes. I also wonder if maybe Maru Botano may have kids cakes.
Nucha has 4 or 5 different locations, Como en Casa is in Recoleta and in Belgrano (Ciudad de la Paz), Maru Botano is in Belgrano (3 de Febrero I believe)
I know of a shop that makes cakes to order. The ones for kids, castles, barbies, etc. But it's in Belgrano in a some what creepy little galeria. I've never had one of their cakes but I often cut through the galeria. So, if you don't find anything else & want to try them. Send me a message & I'll give you the exact address.
Theme cakes usually have to be ordered in advance at confiterias, panaderias, casas de reposteria. Usually they don´t have them on the displays but they have books with pictures. Places like Nucha, Como en Casa, Maru Botana etc, make delicious cakes, they won´t decorate them.
Here are a few I found ( I haven´t tried any of these )
Angie - I have a major sweet tooth and on another expat forum I asked the question about the best postre places in BA. By far Maru is the best. Very fresh and awesome. A fellow expat loved them so much that she had them do her wedding cake. They are abit on the pricey side but the proportions are big and it's worth it. They have a location in Retiro which really is like Recoleta and is walkable, as I've done, from Palermo Chico.