Alicorns aren't born. One of the books confirms that by telling the story of how Cadence became an alicorn. She used to be a Pegasus. So if Twilight and Flash did have a kid, it would be either a unicorn or a pegasus, not both.
Also in that respect, the newest book they released (Journal of Two sisters or something along those lines) Does in fact say that Alicorns are a born race that age slower then Unicorns, Pegasi and Earth ponies =3