I looked at the serrated kids knives online, but not being able to find one to 'see' I just looked in my drawer to find something similar. I started by giving her her knife and showing her how to use it as a proper knife, learning to keep her fingers out of the way. Then i gave her a serrated steak knife similar to the online ones to practice with.
She's quite competent now.
A GF of mine was horrified when I handed her the cake knife to cut her fifth birthday cake this year. She did a great job and served the cake, and then GF's (much older) girls wanted a turn too. DD explained the rules, and all was good. GF was astounded...then proceeded to ask me how she could teach her 8yo to use a knife at dinner time ...just give her one to have a crack! She said but what about the food that goes flying off the plate?....well she'll figure it out.