The key point is that the thrust from the electric motor is not constant. As the speed (revs) of an electric motor increases, the torque (and therefore also thrust) decreases.
Initially the torque/thrust is much higher than resistance, so the toy car accelerates. But as the speed of the car (and the revs of the electric motor) increases, the thrust decreases, and at some point it becomes equal the external forces of resistance, which are approximately constant.
So what external forces of resistance are there? Air resistance remains quite low and insignificant for a toy car. (Unless there is a head- or tail-wind.) As Floris points out, there is also static friction on the non-driving wheels and 'rolling resistance' on all wheels.
While the car is accelerating, the thrust force has to push against the translational inertia of the car but also the rotational inertia of the non-driving wheels which rest on the ground. Static friction from the ground (acting backwards) opposes any acceleration of these wheels. So as Floris states, static friction acts in the opposite direction on driving wheels and free wheels. Friction in the bearings also resists all rotation of these wheels; so when the car is moving at constant speed the static friction force is only required to overcome the friction in these bearings.
Friction in the bearings also resists motion of drive wheels, but technically this decreases the thrust they supply to the ground. (You could draw 3 separate FBDs for the motor/chassis, the wheels/bearings and the road. Friction in the bearings of all wheels would then be external to the motor/chassis.)
Rolling resistance arises from the asymmetry of forces which deform the wheels or ground (or both) when the wheels move. It could also include any tendency of the wheels to stick to the ground.
As garyp says, all of the resistance forces (including air resistance) are usually summarised in a single force-arrow on the FBD, in a direction which opposes relative motion between the car and the ground.