Learn French: D'abord, elle a mangé une barbe à papa ce matin. - First, she ate cotton candy this morning.

Matin is used for a specific point in time. When stating when a distinct event occurred, you use ce matin. Matinée emphasizes the duration or the experience of the morning. Since eating the cotton candy is a singular completed event rather than an ongoing morning-long process, matin is the correct choice.

What does barbe à papa literally mean?

It literally translates to "daddy's beard"! The fluffy texture of the spun sugar gave rise to this fun idiom for cotton candy. Since it is a compound noun joined by à, only the first word becomes plural if you are talking about more than one: des barbes à papa.

Could D'abord be placed somewhere else in this sentence?

Yes, you could say Elle a d'abord mangé... by placing it between the auxiliary verb (a) and the past participle (mangé). Putting it at the very beginning of the sentence with a comma strongly emphasizes the sequence of events, acting like a structural signpost to tell a story.

Could I ever use cette matinée to say "this morning"?