when I have a new child start with me (regardless of age) I set them straight right away....if they are food throwers or plate dumpers I take their hands in mine and in a very serious tone and with my mean face I say "no throwing/dumping" when they hold the cup over the side and are about to drop it I say "no dropping" if they keep doing it then I sit right beside them and catch them in the act and say very firmly no (whatever they are doing) ....at 17 months this child shouldn't be doing that but apparently the parents are allowing it ...sounds like it their first kid .... you don't say how long she has been with you but I would be on her fast ......I would also say to the parents " at lunch time we have been working on eating nicely, not throwing food or dumping the plate ....how is she at home ? and see what they say ? I hate disgusting eaters......