Toys are often supplied with the cheapest battery cells which may be lower capacity, but also have the widest tolerance. This tolerance and the "weakest cell's fails 1st" is the general rule.
To understand this, model each battery as a charged capacitor, say 1000 farads +/- 20 %. Then put them in series with the same initial voltage and load them with, say 100 ohms, and simulate or calculate the end of charge using worst case tolerances, say three cells +20, 0, and -20 %.
Then you will understand. Lithium polymer and car batteries on the other hand start at 0.1% then age towards 10% before single cell death at more than -90% tolerance of the initial charge.
ESR rises and C drops in values towards end of charge rapidly.
If you analyze this, then you will understand.
I pulled the C value out of thin air for simple understanding, but it depends on the mAh hour rating. Ic = C * dV/dt where dV is usually 1.6 to 1 V = 0.6 V depending on chemistry, but the mAh rating is I* dt = C * dV, so 50 mAh = 50 mA * 3.6 ks when rated for some period like 1 C = 20 h, but every and all batteries have some ESR value too, where a wide tolerance applies and ESR generally drops with RISING mAh rating which changes with both low SoC and long age.
The rule of thumb is that ESR is sometimes related to Ah, BUT not a
In the end the ANSWER to your question is the battery cell with the lowest C [F] value DECAYS FASTEST. When in series, if sustained it will also become reverse charged. next question please...
OK here is a comparison of AA battery capacities.
Note above the "DURACELL standard" is 2000 mAh or 2Ah or 7200A-seconds
However in both cases the energy stored is E= ½CV² but in the case of a battery I recommend E=½C(Vi²-Vf²) as Vbat from initial, Vi at 100% Soc to final Vf but rather 10% SoC to avoid deep discharge rapid aging.
Each voltage is very dependent on cell temperature; but as I recall;
if you put a slight load to remove short term memory, the measure Vbat (100%)
- Lipo ΔV=0.7 from 3.6 to 3.0 V
- Lead acid = 12.5 to 11.5 V
- Alkaline = 1.5V to 1V