I've read through a few other answers here on buoyancy and I was hoping to get some guidance on discussing it with at 5th grader.
So, taking a ball of playdoh clay and dropping it into a container of water which is filled to the top that is sitting in another container, just a little bit of water overflows and we can measure that amount using our kitchen scale. She records this as her value to compare other shapes to.
Taking other balls of playdoh which weigh the same amount on the kitchen scale, she's made various "boats" which differ mainly in the shape she's made - some are more cup-shaped, some are more pie plate shaped - some have sides and some are more pizza shaped. The pizza shaped ones sink like the ball did. The ones that are more pie plate shaped end up displacing more water than the others (and certainly more than the ball that just sinks) which are cup shaped, at least for her experiments.
So, she sees that these "boats" are able to float and doing so results in some amount of water that flows over the top of the container but what more can be explained here - that a 5th grader is going to grasp. The weight of the water displaced for each one is different with the cup ones resulting in less water being displaced. Buoyancy isn't my thing so I'm hoping for some layman-friendly explanation as to the difference in the amount of water displaced. And dad will learn a bit along the way too, which is always a bonus.