That is really young, but if she likes it and you have a teacher willing to work with her, why not? I would just be sure that you get a harp with low tension strings (the harpsicle as you mentioned would be nice for this) so that her tiny hands will have the strength to pluck the strings. When teaching really young children, it’s important to make sure that the process of learning is fun.
I myself have seen young harpists (age 6 or so) who had been playing for at least a year or two, so while she is very young, she is not too young I think.
Just my $0.02