Maybe it is an Irish thing that Santa (or Santy) is believed in for much longer.
I think my eldest knows the truth this year (he's 12), but last year he was a firm believer. His sibling (10) queried Santa, but when we didn't rise to informing them of the truth, they gladly let it drop.
Somebody else mentioned the 'Santa bus' earlier. I think, here at least, it's very easy to get kids back on that bus. They may query, but they are very easy to convince to get back on board.
I wonder if there's a greater focus on ensuring kids believe for longer here. All the children in my kid's classes seem to have been told the same thing about the reason for multiple Santas in photos/shopping centres etc - Every now and then, special 'big' elves are born. They are really important because every year Santa is really busy in the run up to Christmas as he has to keep an eye on all the little elves. These big elves go out all around the world to meet boys and girls and collect messages for Santa. That's why they all look different - they're not the 'real' Santa, but they are from his crew.
As for the comment about Mummy paying for presents? I'd not be impressed, but I have heard that before. I'd throw you under the Santa bus in my explanation though - obviously you weren't a very good little girl, and weren't going to get any Santa presents, so your Mummy had to pay for you to get gifts. If you're going to not act with a modicum of cop on, and know when saying less is a good way to go, then tough.
I was 11 when I found out. The thought of my 6 year old 'working it out' and not believing any longer would be a disappointment to me. I see his joy at the 'magic' of everything. The idea that he would be so logical, and such a critical-thinker would not be something I would be happy about at that age.