Well, OP, Elle is a word in French, it means 'She'. It's a pretty short name, but the French pronuciation is "Ell". It can be prounouced "Ellie" as a name, but it won't be by anyone who's studied any french, which as it's taught in pretty much every secondary school in the UK, will be pretty much all of the British population.
sorry, but there it is.
If you want your DD to be called Ellie only, you need to change the birth certificate to Ellie. If she's under a year, that should be a relatively pain free thing to do. If you determined to stick with Elle, you need to accept the prouncation pretty much everyone will use is El. However, the compromise could be whenever you register her for nursery or school, you get the option of "known as" and you can put "Elle known as Ellie" to get round this.
That said, I think Elle (prounced El) is a beautiful and elegant name for a grown woman, and Ellie is a very cute nickname for a little girl, so you could keep it as it is, if you can accept you'll have to say "we call her Ellie" a lot.