Without knowing which passages in the Catechism of the Catholic Church you are thinking of, I cannot comment on them. What I can do is to comment on the idea of telling children stories of Santa, the Easter Bunny, and the Tooth Fairy.
It is not lying to create stories for young children. That is basically what is done when parents tell their children of imaginary creatures such as the Easter Bunny and the Tooth Fairy. From the beginning of time parents have spun such tales for the entertainment and education of their children. Rather than “lying,” it can be considered “myth-making.” Myths are a means of conveying certain truths pictorially rather than didactically. When parents tell their children tales of the Easter Bunny and the Tooth Fairy, the larger truths that are being conveyed mythically are that we are part of a larger universe and are watched over and cared for by good spirits whom we cannot yet know empirically. This can be considered groundwork for later introduction to the communion of saints.
As for Santa, he is based on a real person: St. Nicholas, an early Catholic bishop who is a patron saint of children because of positive interactions he was believed to have had with children during his life. The stories that have been based upon St. Nicholas are what we call legend. Legends are generally non-historical or unverifiable stories passed down through the ages about a person or group of people. While Catholic parents may want to make sure children know about the real-life saint behind the Santa Claus legend, there is nothing wrong with passing on pious legends to children.
The real question probably comes when children find out that their parents are the ones who are providing the Santa gifts, Easter candy, and Tooth Fairy money. In my experience, parents tend to worry too much about how their children will receive this news. Many children through many generations simply accept this information as a part of growing up, and, in fact, will “collude” with parents to keep the myth going by not letting their parents in on the fact that they know The Truth in order to avoid spoiling their parents’ fun. But, if a child does feel betrayed to find out The Truth, then the parent can explain the context of storytelling and myth-making, perhaps pointing out to the child that Let’s Pretend is a game for people of all ages.
Of course, it should be said, if a parent does not feel comfortable taking this approach to Santa, the Easter Bunny, and the Tooth Fairy, the parent is free to leave out such stories from his child’s education. He should though teach the child to be sensitive to not spoiling the fun of other children by telling them that such characters are Not Real.
Yes, Virginia, There Is a Santa Claus by Francis P. Church