My buns love sow thistle too, prickles and all! The common names of sow and milk thistle seem to be mixed up some times, but milk thistle (Silybum marianum) is a different species in the same Family (sow thistle is Sonchus arvensis).  I see reports of people feeding it to bunnies online, but seems to be mostly for some medicinal properties on the liver. I also see (on wikipedia) that it is toxic to cattle and sheep due to high nitrate content.

To make sure you get the seeds you want, try searching with the scientific names to avoid the confusion between the common names. My impression is that its not toxic, but with the medicinal properties and high nitrates I wouldn’t feed it as a staple?

Dandelions are very closely related to sow thistle and the seeds are easy to find, so might be a better option to grow at home. 🙂 Narrow and broad-leaf plantain is another easy to find and grow edible weed.