There is no one size fits all answer to the question. As a starting point, the question is whether what OP wants is even possible. On a fully-booked flight, the simple answer may be that there are no seats available, so there is nothing to be done.

Other issues involve the GA's time and other distractions as well as the importance of the issue to OP's family (are we talking about 3 small children under the age of 5 or 5 adults, is some disabled and requiring care, is the GA tied up handling a reroute for another passenger and then boarding the flight)?

You can ask at check-in, the gate, UC (if you have access), a FA and then resort to self-help by asking other passengers and being prepared to offer up "better" seats in order to get yourselves together.

In the end, unless there are little kids or disabled people involved, being seated apart won't be so bad.