  The recent discovery of a new population of ultra-high-energy gamma-ray
sources with spectra extending beyond 100 TeV revealed the presence of Galactic
PeVatrons - cosmic-ray factories accelerating particles to PeV energies. These
sources, except for the one associated with the Crab Nebula, are not yet
identified. With an extension of 1 degree or more, most of them contain several
potential counterparts, including Supernova Remnants, young stellar clusters
and Pulsar Wind Nebulae (PWNe), which can perform as PeVatrons and thus power
the surrounding diffuse ultra-high energy gamma-ray structures. In the case of
PWNe, gamma rays are produced by electrons, accelerated at the pulsar wind
termination shock, through the inverse Compton scattering of 2.7 K CMB
radiation. The high conversion efficiency of pulsar rotational power to
relativistic electrons, combined with the short cooling timescales, allow
gamma-ray luminosities up to the level of $L_\gamma \sim 0.1 \dot{E}$. The
pulsar spin-down luminosity, $\dot E$, also determines the absolute maximum
energy of individual photons: $E_{\rm \gamma~\rm max}\approx 0.9 \dot
E_{36}^{0.65}~~\rm{PeV}$. This fundamental constraint dominates over the
condition set by synchrotron energy losses of electrons for young PWNe with
typical magnetic field of $\approx$100$\mu$G with $\dot{E} \lesssim 10^{37}\
\rm erg/s$. We discuss the implications of $E_{\rm \gamma~\rm max}$ by
comparing it with the highest energy photons reported by LHAASO from a dozen of
ultra-high-energy sources. Whenever a PWN origin of the emission is possible,
we use the LHAASO measurements to set upper limits on the nebular magnetic
field.
