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.
Learning music is just like learning a language. The earlier you start the better, but it does not actually have to be with an instrument in the beginning stages. Very young children (2 and younger for example) are able to learn music, although the harp is difficult for very young children in terms of manual dexterity. Studies have shown that learning a musical instrument from a young age can improve overall memory, improve scores on standardized and iq tests and they also show improved listening skills. Their shorter attention spans demand a teacher with a great deal of patience and very short frequent lessons. If your child is interested in the harp and the teacher is willing, I would go for it!