With William Robert:
Julia Elizabeth or Kate
It would be nice if they both had a family name somewhere.
If you wanted, Lena could be a nn for Elena, Helena, Magdalena, etc. Mila could be a nn for Camila, Emilia, or Milena as a previous poster suggested. The names are fine on their own. They just feel a bit nickname-y to me next to William. I think William and Rose would be cute siblings.
I love Mila Rose. It's just perfect and works well with William Robert.
I like Tessa. Perhaps Tessa Katherine, Tessa Rose, or Tessa Madeleine.
I can't really comment on Lena or Julia because of people I knew with those names...
Some other names I would suggest are Anya, Nova, and Camila.