I would ask about staff retention. How many of their teachers have been there for 4 years or more? How many staff left last year?
If class numbers are less than 30, ask about the trend in that over time, and also look for evidence of the lower budget that comes with.
Ask about who covers the classes during a teacher's non-contact time and for absence - is that person a teacher or a teaching assistant? Equally, if any classes are job shares, ask whether both parts of the share are teachers (increasingly common for schools to use a teaching assistant to cover, including for multiple days a week).
Enquire about their provision during Covid lockdowns, not because that is likely to arise again but because it will give you a really good idea about the ethos of the school - did they focus on education, on pastoral care, on supporting the vulnerable? Ask what gaps they have found post-Covid and how they are dealing with them.
In mixed-year classes in particular, ask for examples of how they challenge the most able, and also how they support those who find things more difficult - the spread of ability and curriculum in mixed year classes is higher than in single year classes, and while e.g. an able child in the lower year will tend to be extended through the presence of the older year, the next year when they are the older year can be one of some stagnation.
Overall, observe the children and the interaction between them and the adults. Are the children cheerful, engaged, respectful of adults but smiley in their presence? Are classrooms busy but orderly, or very silent, or a bit frantic?