How can I limit the current in a 12v car battery to be safe for my 25a motor driver?
The motors are 24V and consume more than 20A in your robot. This is a fact and it is also a fact that the motors are likely to draw more like 30A to blow the fuses in a few tens of seconds and possibly 40A if the fuse blew in a split second.

Limiting the current to the motors can be done to prevent the fuse blowing of course but you have to decide what you want: -


Do you want full power available from your motors or
Do you want significantly less than full power due to current limiting?


Be also aware that simple current limiting with a series resistor of say 1 ohm means that when the motor is taking 10A, the power dissipated in the resistor is \$I^2 R\$ or 100 watts. That's a pretty big resistor and a lot of power wasted.

A better way is PWM control of the motor so it can't exceed a certain current level. This can be achieved with just a couple of watts dissipated but again you have to decide what you need for your robot and if, in fact, you are using the wrong sort of motors for your application OR whether your sabretooth controller is not man-enough for the job.