I Want To Make My Daughters Birthday Cake This Year. But Have No Clue Which Way To Decorate It?

It sounds that your daughters would be twins if they are having one cake. What i have done with two other people that share the same birthday with me is we got a 3 layered cake. Each layer was decorated with separate things that we liked.
The bottom layer was covered with frogs.
The middle layer, which was mine, was brown w/ teal and pink polka-dots
the top layer, which was for the girl turning the youngest had some other decorations.
Each layer could also be different types of cake if they don't like the same thing. That is what i would do.

I need to know how old she is. Blurt back with more details, and I'm sure you will get a lot of ideas.

Do  what they like on it  they will love it!