Originally Posted by jebr

Out of MSP the difference between basic and main is often $30-$35 per person each way. For two people round-trip that's over $100.

As for basic economy for a family, the biggest question is whether the 9-year-old would be okay sitting alone if there's no seats next to each other. While checking in at T-24 will help, and the check-in desk and gate agents may try to pair them together, if it's a full flight they may still be separated despite doing everything they can. If that's an unacceptable risk, buying a main cabin ticket, selecting seats on booking, and checking every Sunday to ensure there's no equipment swaps (or that, if there was one, that seats were re-assigned correctly) is the best way to ensure that the seats are next to each other.