Art my youngest son Charlie did a few years ago. Tile for this art project provided by Portland Parks (through their summer activity program). 895 ppm lead. Tested with an XRF & Posted, 3/17/2016.
The amount of lead that is considered toxic in an item intended for children is anything 90 ppm or higher in the coating (glaze/paint) or anything 100 ppm or higher in the substrate.
Given this art project just consisted of markers being used on the surface of a tile that has been glazed and fired at very high temperatures, I don’t have a huge concern about something like this in general, although I don’t think it sets a good precedent (to use leaded tile of any kind for art projects for young children.)
As always, please let me know I you have any questions.