Resign - accept that something undesirable cannot be avoided.

Dad resigned to his seat. "Fine," he says.

Relent - cease resistance.

Dad sits back in his seat and relents. "Fine," he says.

Capitulate - cease to resist an opponent or an unwelcome demand.

Dad sits back in his seat, capitulated. "Fine," he says.

Acquiesce - accept something reluctantly but without protest.

Dad sits back in his seat and acquiesces. "Fine," he says.

Resign - accept that something undesirable cannot be avoided.

Dad resigned to his seat. "Fine," he says.

Relent - cease resistance.

Dad sits back in his seat. "Fine," he relents.

Capitulate - cease to resist an opponent or an unwelcome demand.

Dad sits back in his seat, capitulated. "Fine," he says.

Acquiesce - accept something reluctantly but without protest.

Dad sits back in his seat. "Fine," he acquiesces.

The way you posed the example sentance it sounds like you're looking for a verb?

Resign - accept that something undesirable cannot be avoided.

Relent - cease resistance.

Capitulate - cease to resist an opponent or an unwelcome demand.

Acquiesce - accept something reluctantly but without protest.

Resign - accept that something undesirable cannot be avoided.

Dad resigned to his seat. "Fine," he says.

Relent - cease resistance.

Dad sits back in his seat and relents. "Fine," he says.

Capitulate - cease to resist an opponent or an unwelcome demand.

Dad sits back in his seat, capitulated. "Fine," he says.

Acquiesce - accept something reluctantly but without protest.

Dad sits back in his seat and acquiesces. "Fine," he says.