A fascinating area of research has been following children's response to an invented fantasy character, the Candy Witch, who is kind, visits at Halloween, and gives you toys if you leave out some sweets for her. For younger children (4-5-ish) generally children tended to believe she was real - the more other make-believe figures they believed in, the more likely they were to believe. Children didn't need "backup" in the form of a parent-organised "visit" if they believed in a lot of other fantasy figures.
By the time children are 7 or 8 few of them believed in a lot of other fantasy figures (Father Christmas, the Easter Bunny, the Tooth Fairy etc.), but if they did, and they got a "visit", they were more likely to believe in the Candy Witch. It seems as if, should parents wish to keep the belief in fantasy figures alive in older children, they are going to need to put a lot more effort into it, and provide a lot more "evidence" that Father Christmas really exists.
Although some people (Richard Dawkins, I'm looking at you) think that children are really quite credible, and will believe more or less what their parents tell them, without any evidence, in fact children do seem to need evidence - they are not as credible as some people would like to think.
If you're interested: