Should I tell my kids that Santa is real after their dad told them he is not?

My ex-husband told my two adolescents that Santa doesn't exist on the day before Christmas. They are desperate to hold on to the magic of Christmas, but I don't know if it's right for me to lie to them and tell them that Santa is real. I don't want to let them down and make them lose out on those memories, but I also don't want to lose their trust.