To avoid buying 24 advent calender presents times your number of children, why not make another type of calender where the present is a happening; a visit to the swimming pool, a dinner at own choice, a visit to the playground or a café, a playdate with the cousins, making a cake or just anything. You could use 24 envelopes like this one.
Luckily in Iceland we have the custom where the 13 Icelandic Santas visit the children that are nice the last 13 nights before Christmas. They are leaving a small present in your shoe if you put it in the windowsill when you go to sleep. So parents save a lot of presents until the day they come to town.