Any suggestions what theme you will give on the 1st birthday party of twins boy/girl?

One can never go wrong with Minnie and Mickey Mouse. Although using this theme would mean you have to stick with red, pink and black instead of the traditional pink & blue. If you prefer to use the pink and blue motif, you can focus on those colors as a theme. With this, you can use pink ribbons and blue bowties as your symbols and you can incorporate this with the cake design, invitation, centerpieces and decors. Hope this helps!