Now i want to control the small motors by using this relay shield. Is there any danger in doing it that way? (could save me the money of buying a motor controller)
If the relays can handle the current of the motor(s), which they probably can (I don't know what relay board you're thinking of), The only drawback is that you only have full speed or no speed.
If you can follow a schematic and solder together a few components, you can add speed control to the relay board, but you'll need an I/O line for controlling each (assuming 4 circuits).
Each wheel has got an encoder, so you need four more I/O's to read them.