I have been asked to do a cake for 3 children who will be dressed as catgirl, superman and batman. In addition to cake (no ideas yet) she wants a separate cupcake or small cake for each one. How do I decorate those? any ideas?

I'm not very good in english (not my main langage) so hope there aren't too many misakes...
When I read your description, I'm thinking three tier cake with each one in the colors of the super hero and with the logo of each on the adequate tier. I would do the bottom one for batman (with the bat logo) then superman (with the "S" logo) and then finish with catgirl and there I would do her mask either aroud the tier or as a topper.
For the cupcakes I would also do the "logo"

Otherwise you could make a single round cake, with skyscrapper views around it and do a simplified model figurine of each caracter. (a bit rounded and just dressed in the outfit)

Juste my thoughts of a new hobby caker HTH

Ps 1: found in the gallery an exemple of a 2 tier cake
http://cakecentral.com/gallery/2414304/super-cake

PS 2: and found an exemple of the figurine I would sugest doing. here it's batman and it's in wood but I would do it in fondant a bit of tylose added or a 50/50 fondant gumpaste mix (don't have that here so i found tylose)
http://www.google.be/imgres?imgurl=http://aweddingcakeblog.com/wp-content/uploads/2011/07/batman.jpg&imgrefurl=http://aweddingcakeblog.com/2011/07/cake-topper-friday-batman-wedding-cake-topper/&usg=__UL71ceCkOKvc9tcPm0yPPZv8l9w=&h=638&w=570&sz=63&hl=fr&start=17&zoom=1&tbnid=1Vyujrq1L__vmM:&tbnh=137&tbnw=122&ei=vz9xUPLDOMnC0QX92IHACA&prev=/search%3Fq%3Dcake%2Btopper%2Bbatman%26um%3D1%26hl%3Dfr%26gbv%3D2%26tbm%3Disch&um=1&itbs=1

thanks so much. Batgirl was not as popular as Batman so could not come up with an idea for her. So appreciate your ideas.

glad I was to give you a little help.

In your first post you mention catgirl in the second you mention batgirl...
if it's catgirl I would would use the color black and pink like her costume and do her mask with pink iner ears.

http://www.google.be/imgres?imgurl=http://images.wikia.com/marvel_dc/images/b/bf/Catgirl_01.jpg&imgrefurl=http://dc.wikia.com/wiki/File:Catgirl_01.jpg&usg=__RxQVu80HDSAdsybI7ptd1OpyX38=&h=655&w=377&sz=217&hl=fr&start=5&zoom=1&tbnid=DEJ1MaOUghpZ_M:&tbnh=138&tbnw=79&ei=tqVyUJG9FIXL0QWKjoCIDg&prev=/search%3Fq%3Dcatgirl%2Bcatwomen%26um%3D1%26hl%3Dfr%26gbv%3D2%26tbm%3Disch&um=1&itbs=1

if it's batgirl since she is a red hair i would use that to make the difference with batman (do the mask with red hair coming out?) and she has the iner side of her cape yellow (were as batman has it black and in the comics his costume is greyish)

those are the differnces i've seen while I googled those names...

HTH

I am sorry, as I reread the mom's post it does ay batgirl.

don't be sorry (hope my message didn't sound rude )
No it was just because in my first post I gave you ideas for catgirl so not knowing in the second i gave you ideas for both.
I think to make a difference between batman and batgirl I would go with the colors of batman in the comics (a sort of blue grey) so you could go black and yellow for batgirl (and i think I would try to incorporate somehow the red color of her hair... )

thanks much. Appreciate it. I saw a batgirl mask with the batman insignia but it was done in pink and black.