When teaching Web Development, should server or client side come first?
Starting with frontend does not necessarily mean starting with JS. You could first do HTML and CSS, which is engaging, as you mention. Then proceed to Ruby for the backend (whether Ruby is suitable as a first programming language is another debate), i.e. generating static pages dynamically.

JavaScript could come at the end of the course. This is also how things went historically, more or less. I would say this order makes the reasons for JavaScript (faster and smoother interaction, a.o.) stand out better.