Is it morally reasonable to mislead or lie to a child for his or her own well-being? Is this sort of parental behavior a necessary "tool" for guiding a child to do the right thing until they can figure out what is right or wrong on their own?
Many cultures use the idea of Santa Claus making a list of who is naughty/nice as a way to manage the behaviors of children (because those children on the "nice list" receive presents). Do you feel that this parental behavior is morally reasonable?
I will use an example to elaborate my answer. Assuming that the man you are bringing up the child with is not her real father. In her early stages it is just right to let her recognize him as his real father. this will help her not to feel out of place alongside other children which would lower her self esteem and also for her to give the man his respect like a dad and not an intruder.