Many thanks to everyone who responded with advice! I really appreciated your suggestions, especially about pre-drilling. I had never used a drill before, and this was a basic project, but it was a lot of fun to complete with my 6 year old son. He did tell me that some of my screws were not 100% flush, so I definitely could use more practice.
A photo of the finished bed before I installed in in the garden is here: https://www.pinterest.com/pin/390968811384141038/
Looks good Jody, and you're now on the way to more DIY projects and even more fun!