The music concept for the class this day was A-B-A form ("same-different-same").
The kids get to play instruments from each group as the summer goes on. Today was Irish Harp. Later that day when his dad asked him what he did in music Spidey said, "We played the Maraschino Harp!"
That old "sh" sound occurs in a lot of words. It's easy to make a mistake like that.