The OP asked have any modern artists done Child Ballads or other trad stuff. It then gave first charted in this century and would be known as examples of what type of things are being looked for. That doesn't mean some of the examples are totally irrelevent so it'd be a crime to mention them. Both Roberts and Mitchell are young and perform said material so whilst they don't hit the OP criteria in some respects they do in others. Of course they are not well known outside their genre but just because kids don't know about them it doesn't mean they shoudn't or couldn't! If we're talking specifically pop/rock artists then for really young kids even really recent performers are old news. The nearest I can think of just off the top of my head would be Natalie Merchant who did a folk album with some trad songs on it including "The House Carpenter's Daughter" and "Wayfaring Stranger" though her solo recording career started in the mid-90s so not quite within the time frame. But let's face it for very youngs kids Natalie would even be a fogey - so even we're talking solely people who are in the pop charts at the moment then yes it is a pretty barren field. There is Rhiannon Giddens of the Carolina Chocolate Drops who have charted in the US album chart be it the lower reaches and to be fair any folk artist who has charted is probably going to be in the lower reaches. So do we mean big international pop stars who sing trad songs - then it is going to be pretty slim pickings. Perhaps Susan Boyle but she'd be as relevant to young kids as Alexander Armstrong would be.