Hacked by 'Erreur 404' – can anyone help please

I’m very new to WordPress and loving it.

I have created 5 sites but one was hacked as soon as it was launched. A second followed shortly afterwards https://www.deloraineshow.com.au. The first one I worked on for a week, using a backup, but it’s still broken. The Deloraine Show one I haven’t touched yet, because obviously I don’t have a clue!

I don’t know whether it’s significant or not, but the two which were hacked use the TwentyTen Allure Child Theme. None of the ones which are still OK use this child theme.

Could someone please have a look at the site and give me some help to get it mended? I’m really desperate here (and hoping to learn for the future).