I used to have very long hair, & now my DD does. To preserve moisture, ideally you need to use as little of the surfactants (cleansing chemicals in the shampoo that break up the oils) as possible, especially on the length of the hair. I always thoroughly massage a non silicone, non protein conditioner into the length, wash the roots, rinse the suds through the length but don't actually wash it, then condition again. You can condition the roots as well if they are dry - the right conditioner will remove the majority of grease & dirt. However for this to work, your DD needs to stick to silicone free products the rest of the time (as 'cones will build up & cause lank, dull & even sticky effect if not removed by the right surfactants, primarily SLS or SLES which are quite harsh on the hair). Follow with a nice oil, like you use, applied wet - if she's resistant perhaps add some essential oil to fragrance it a bit & make her hair smell beautiful?
Oh and, if your DD can bear it, lay off the heat occasionally. Regularly frying the hair permanently damages the hair shaft, which means the individual hairs cannot retain moisture as well any more.
It works, honestly. My hair used to be nearly knee length & very thick with almost no splits. DD's hair is past her bum with no splits at all (she is 5 so she regularly comes home with sand, paint etc. in it, I just do the best I can with the no shampooing the length!)
For splits - the best thing to do is cut them out. Either by a general trim, or, if you / she can bear it, individually with something like nail scissors. It takes hours of your life but you can retain a lot more of the length. Perhaps in front of the TV...