To be honest, I am not really sure what the best sort of plugin would be.
The fundamental point is that all the data is sitting there in the database, and Moodle has functions to help you load it. Then, only you have loaded it, you need to output it in some format. I would acutally suggest CSV, rather than XML, because it is simpler, and partents can look at it in Excel.
Then you need a plugin to package up the code. I wonder if the best thing would be a block, just because they are an easy sort of plug-in to write. Then, the adminstrator can add this "Download my child's grades" block as a sticky block that appears on every user's user profile page.