The specific job and field determine how flexible flexitime really is. And whether there would be collective gasps of horror if you tried to leave on time. Or were expected to work in the evenings or weekends at home.
Some places allow you to "buy" additional holiday or book unpaid leave, so that may be useful re childcare. Or have various childcare schemes in their benefits packages.
Again depending on the job you might be left to manage your own time or be expected to log every single thing you did all day and how long each took. You might be given an allocation of time for tasks that is insufficient but still be expected to complete the work. Or it might be less rigid. You might have to think about billing or how commercial it is for you to spend a length of time on a task. You might be asked to justify the profit or loss they made on your work.
Offices tend to have a window of time (e.g. 12-2) where you have to take your lunch break but leave it up to you to decide each day when to take it. Maybe a rule on how many people can be at lunch at once. If they don't tell you during your induction ask.
Booking holidays tends to depend on who else is there to cover. Some places with big teams will be happy as long as someone in the team is left and deadlines are met, other places buddy you up with someone so you can only book holiday if they haven't. So you need to plan ahead and possibly negotiate with colleagues on who will be off when.
Some places will have a flexitime rota to ensure that there is always someone in the office for the early/late slot, so you may not have the unbridled freedom you expect. Also ask about that.
Or an unwritten rule that nobody is supposed to actually use flexitime other than for emergencies.
Is it proper flexitime where you can build up time to book off later or does it reset each day but just give you flex on what time you start (and therefore finish)?
Does unused holiday carry over to the next holiday year or is it lost if you don't use it? Do they close for Christmas? If they do, are you obligated to use a certain number of your holiday days to cover it?
Policy on emergency leave (booking annual leave on the day to deal with an ill child) or would you have to take unpaid leave?
After a certain period of service will your holiday entitlement increase?
So many variables...