I think two brief sentences is probably fine, but it wouldn't hurt to work on really "selling it." For instance, one of my experiences that I included was a volunteer opportunity where I set up a room with snacks and toys and watched/babysat kids while their parents attended an AAC workshop. Instead of writing that previous sentence as my description, I wrote, "Monitored and played with children on the autism spectrum while caregivers attended Augmentative and Alternative Communication workshop. Created an environment that stimulated learning by setting up room with snacks and toys. Assisted children with eating and other daily tasks to ensure a safe environment." As mundane as a few of your experiences might seem to you, I bet if you really think about it, it gave you skills or experiences grad schools would see as valuable.
I found it helpful to continually go back and see if I could improve what I had written. Try looking up a list of powerful action verbs for resumes online and see if you can use a few of them in your descriptions.