It requires a detailed historical backgrounder beforehand (WWII, Nazis, evacuating children from London, etc.) and sometimes during. Also, everyone has an English accent, and several are quite thick (including the children). My kids didn't fully understand many of the main details: that Ms. Price isn't their mother, that the village they're sent to isn't London, etc.
That said, they mostly enjoyed it. They laughed at the soccer game on the Isle of Naboombu and the final battle against the invading Germans. My 7yo was initially very frightened when Ms. Price looks out the door and sees Nazi soldiers staring at her, so here's a pro tip: after Mr. Browne leaves for the train station, right when Ms. Price starts to put the cat out for the night, pause it and warn younger children what's about to happen.