A.B.
I actually just went to a very successful 5yr old bowling birthday party. It went great. Lots of younger siblings too. I think to set the expectation to - nobody stay in any specific lanes (maybe have a cone on each side, to frame the reserved lanes), and no scores - is a good idea. We got there late, but the grandparents who were keeping score quit 10mins into it. The kids had fun just rolling the ball down the lanes, and watching anything get knocked over. have fun!
The structure was mostly: Playtime, Pizza Time, Cake Time.