The description of the death of Ettiene, is that the care blew up in the car port, and when the mother tried to get to Ettiene in his bedroom, there was no bedroom to go to, but the boy was on the lawn in front of the house. Louis Daniel, the father, says the mother hangs out in the boy's room, but the father cannot manage to go in. At the end of the episode, Louis comes home, and the mother is in the boy's room, and he joins her. The room in entirely intact. How can it have blown up, and be intact?
The LD50 of Polonium 210 (acute lethal dose for 50% of the populace) is only 50 nanogram, very tiny grain - so it wouldn't require a big vial nor leave a noticeable taste in a full glass of wine. But more problematic is that it can't be detected with a Geiger counter at all, although it's a central part of the unmasking. A study of the Litvinenko case, which inspired this episode, would have learned that: since the Geiger tests of the patient were negative, his radiation poisoning remained a mystery for some time.