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... is very easy to obtain, even for a child.

Sampling breath can be done by using a straw or using an Breath Sampling Device.

The breath sampling device is used for taking breath gas samples in
Tedlar bags under controlled conditions. Here this device is called in short a
“Breathsampler”.
The Tedlar bag is positioned inside a vacuum box, which is evacuated by a vacuum
pump. The breath gas is sucked into the Tedlar bag by subatmospheric pressure.
The sampled exhaled breath flows through a heated Teflon-tube connecting the
mouthpiece holder and a heated magnetic valve into the Tedlar bag. Heating avoids
condensation.

The differential pressure-sensor inside the mouthpiece holder delivers the data
necessary to calculate mean duration of exhalation; from these data the time for
opening the valve in the following exhalations is determined. During the first 3 sec. of
an exhalation the valve will never be opened. To get a constant and reproducible
result, the breathsampler does not take a breath sample from the first exhalation.
This ensures that only breath gas from the alveolar phase is taken. As soon as the
Tedlar bag is filled, breath gas sampling can be terminated. The device does the
restart of the breathsampler for the next breath sampling procedure automatically
when no breath gas flow is recognized by the sampler for about 15 sec.