Spend an hour or two on the site with a budget in mind for every child in your group. Even if it is $5 or $10, any amount will help. They can search through those in need and split up their budget or donate to one. No, this is not a "whoo hooo, let's have fun" activity, but rather one that is heartfelt and hopefully helps not only the children but yourself feel as though you are helping someone relieve pressure off of their illness from a monetary standpoint.
Since some of the illnesses are things your kids may have never heard of, look them up. Knowledge can build compassion. Look at what happened when someone developed the ALS ice bucket challenge, a disease very close to my own heart. So many people now realize what that disease is. The research from the monies raised can really bring about a difference to a cause.
Want to step it up? Instead of giving them an amount to donate, have them sell one of their favorite toys and use that money to donate. You can do it on your social media outlet by auction style.