He didn't lie, he just used misleading word choice.
The Doctor did have a plan: he had a clear expectation of what was to come and the across he would take. That plan just happened to be bunk, but not enough bunk to be a lie when said to a child.
Clara, on the other hand, was an adult and companion, for whom the Doctor having a plan carried the very specific connotation of a way to resolve the current crisis in an acceptable fashion.
And, of course, this entire theorum assumes that (1) the truth field was still in effect after so many centuries, (2) the field was compelling truth instead of barring deception, and (3) there wasn't just a specific exclusion for children within the field.
(It'd be a pretty lousy childhood in a town called Christmas if no one could ever tell you tall tales, ghost stories, or the legend of Santa Claus.)
(Not to mention that the strength of the truth field varries by location, and long exposure seemed to reduce its efficacy.)