I think this movie (which is otherwise very good) is really off for the pre-K/K set and iffy for 1st-2nd grade. The site review implies that the only issue is some cat/mouse scariness, but in fact the whole movie is, as the BBFC rating says, "tonally dark".
The site review says that a minor character dies early on; in fact this character is the princess's mother, the incident of her death is revisited throughout the film, and the human characters grieve deeply for her. Adding to how upsetting this event is, the movie does not provide an obvious cause, which can give the impression to a young kid that their mother can just drop dead.
The mouse (Deveraux) is banished to the dungeon for breaking a rule (talking to a human) that was not specifically articulated to him; the expectation is that he will be eaten by the rats (so it is de facto a death sentence). His parents just sit helplessly watching this, crying in their hankies but doing nothing to interfere.
A rat which is a "good" character for most of the movie does something truly awful; a peasant girl quite delusional (i mean mental-illness delusional, not just has fancy ideas); the villain is definitely not played for laughs; there are "gladiatorial" scenes were victims have to fight for their lives; etc.
This is a good movie for kids that are able to handle dark material, and raises some very good issues about the complexity of human behaviour and especially grief. It is good watching for adults, but approach it carefully and be prepared to spend a lot of time discussing it.
Frankly, because of the persistent dark tone, I wouldn't even have rated it G, I think it is a PG emotionally even though it lacks a lot of the "tickboxes" like violence, language and so on.