How strong can i expect wood glue to hold
Better wood glues make the claim that your joint will be stronger than the wood itself. That's to be taken with a grain of salt, as it really depends on the application. It's often true that a broken joint actually breaks off a layer of the wood rather than the glue itself. 

In this case, if you glue the joint thoroughly I'd expect it to perform as expected, though the thickness of the side rails (and therefore the joint surface area) is a factor in ultimate strength. Unless the tray was dropped or otherwise impacted it will probably hold up well. That said, it might be wise to run a single small finish nail (with or without its head) through the joint to reinforce it. If you're going to stain it dark it's easy to hide something like that.