I've found design critiques are what separates the 'artists' from the 'designers' more than anything. A designer creates work suited for a purpose, so if the design they have created doesn't hit the target, it's not a comment on them as a person just a comment on the design.
You have valuable information (knowing how to improve the design) and so you need to frame that news as a good thing. The design will be stronger for it if they listen.
Another thing you can do to help protect people's feelings when talking about design changes is always keep the design the focal point of every sentence and speak about every change from the design's perspective, not the designer.
“You put this too far to the right” should become “This feels like it wants to be further to the left”
“This is sloppy and you didn't put things in the right spot” becomes “This layout feels unbalanced, and if we moved this over here I think it would be more stable”
“Why didn't you make this bold like the others” becomes “This one feels like it wants to be bold too” etc.
Hopefully with a little luck and grace, they will listen to you and be grateful for your input! Even when I have done my very best on a design, I enjoy hearing insights others have; iron sharpens iron!