Adding a bulb prevents things happening
Wrong Track: The first bulb grabs the current, so that there's none left for the second.
Wrong Track: The first bulb slows down the current, and the second one slows it even more.
Right Lines: The current is the same everywhere in the loop. The bulbs both act together to reduce the flow. There really isn't a
One more bulb, reduced current
Thinking about the learning
The fact that adding a second identical bulb in series results in the two bulbs being dimmer makes intuitive good sense to many. Their underlying reasoning is that the single source (or battery) is now being shared between the two bulbs. This idea needs both development and some refining, so that it increases in precision.
The key idea is that adding an extra bulb introduces more resistance to the circuit, and this has the effect of reducing the flow of charge in every element in the circuit. The number of charges passing any point in the circuit is reduced; in other words, the current is reduced.
Thinking about the teaching
The rope loop teaching model is very useful in exploring the idea that the electric current goes down when a second bulb is added to the circuit. A discussion for later might go like this:
Teacher: OK! Now I want both Julia and Anita to loosely hold the rope. I'm pulling the rope round with the same force and it's obvious that Anita's extra grip or resistance has slowed down the whole of the rope loop.
Having talked through the teaching model, attention is returned to the electric circuit model:
Teacher: When a second bulb is added, extra resistance is introduced to the circuit. This has the effect of slowing down the flow of charges all around the circuit. In other words the current is reduced.
It is worth emphasising that it is the filament of the bulb that offers the resistance to the flow of charge:
Teacher: The filament of the bulb is made from very thin tungsten wire. The filament is designed so that it is difficult for the charges to pass through and, as they do, they interact with the tungsten atoms and the thin wire heats up until the filament glows white-hot. This is rather different from what happens in the connecting leads. These are made from relatively thick lengths of copper wire that have a very low resistance and so very little heating occurs in the connecting leads.
For now, it's enough that the explorations of circuits don't put tripwires in place, to catch the unwary later.
The fact that adding a second battery in series results in the bulb being brighter makes intuitive good sense to many. The reasoning is that the double source (or battery) is now supplying just one consumer (or bulb) and therefore gives a bigger effect.
A more detailed explanation is, however, more demanding and is based on two effects which occur simultaneously:
With an extra battery, the positive terminal of the battery becomes more positively charged and the negative plate becomes more negatively charged, and this exerts a bigger force on the charges.
As a result:
In other words, adding a battery both increases the current and increases the effect of each charge passing.