"My pizzas, why don't you try them?"
Translation:Mis pizzas, ¿por qué no las pruebas?
I don't think so. Generally, when try means sample, taste, test its probar. Tratar is used more for attempt to do something.
Why don't you try them, the them is the direct object (answers the question what is being tried). It also is feminine plural for pizzas, so DO fem pl = las.