I think that as you own the business and you benefit from the profits & they are just employees, you will never get the same amt of committment from an employee.
So although it may feel unfair that you do 60 hour weeks, you can't really use this as an argument.
However if you are genuinely paying them generously for the industry, and asking, rather than telling, them when extra work is needed, you are treating them quite well. After all in the current climate who can afford to turn down extra work? Not many I would think.
Perhaps you could take on a temp or two for Christmas, get them to do the awkward shifts, and not give the others extra work. If they query this you can say "I need XY and Z shifts covered, and as you have all refused, I have had to look elsewhere". This may result in a flurry of extra effort from your long term employees (but could cause conflict). However although change can be difficult, imagine if you got an ideal employee through the temping, and one of your poorer performers decided to leave? You could end up with a nice pool of temps to pick from for extra busy times too.
Or, you could do a set rota (ensuring there is sufficient cover for the days you need extra people) and inform them that if they can't do the rota'ed shift, they need to agree a swap with someone else (this may not work if you have to put them in for more hours than set out in their contracts).
I do think they are being a bit cheeky - after all, I'm sure they are spending more at Christmas than any other time of the year, so it seems they are looking for the extra hours for exactly when suits them, rather than when it suits the business.