I don't know that I ever believed. Even when we were keeping up the pretence I didn't actually beleive it; it was just fun to talk about Father Christmas coming in the night. It was a thrill even though I knew it was my parents all alomg
Neither of my children have ever believed in Father Christmas. For one thing I don't believe in lying to my childred. How can I, in all honesty, teach them that lying is wrong when I have been telling them such a big one for so many years. For another thing I am not Christian and therefore don't celebrate Christmas as such. We celebrate the Winter Solstice/ Midwinter Festival, a few days late. Fortunately most of the traditions are the same it's just the core meaning that's different.
For us the true meaning of christmas is a little darker. The red and white colorus, the robin, the berries... symbolise blood on the snow; At the Winter Solstice the Oak King is cut down by the Holly King who marks the ultimate dominion of winter. However, where the blood hits the snow a seed is born that lies dormant in the snow and grows again in the spring maturing again to the summer solstice when it is born as the golden child who grows into the oak king.
It's a lot more complicated than that and, of course, it's all symbolic. but that's what we do. The children have always known that the 'spirit of christmas' who some call Santa or Father Christmas, and who is seen sometimes as a fat man in red, sometimes as a saint, sometimes as a hook nosed witch... no matter how it is seen it's gift is the promise of new life which lfts spirits and makes people nicer to each other for a time. (generally)