What ages did you let your children use these at? Are they a nightmare/impossible to get off tables, hands etc? My 7yr old loves watching YouTube drawing classes which seem to use black markers around drawings. He’s desperate to use a sharpie having used at friends. Problem is he draws in livingroom mainly and has a 3.5yr old sibling so I’m not overly keen to have permanent markers. We use crayons washable pens mainly but he’s gone through all the black ones. Any suggestions. Thanks