  Newborn, strongly magnetized neutron stars (so-called magnetars) surrounded
by their stellar or merger ejecta are expected to be sources of
ultrahigh-energy neutrinos via decay of mesons produced in hadronic
interactions of protons which are accelerated to ultrahigh energies by magnetic
dissipation of the spindown energy. We show that not only pions and kaons but
also charm hadrons, which are typically neglected due to their small production
cross sections, can represent dominant contributions to neutrino fluence at
ultrahigh energies, because of their short lifetimes, while the
ultrahigh-energy neutrino fluence from pion and kaon production is suppressed
at early times due to their significant cooling before their decay. We show
that the next-generation detectors such as Probe Of Extreme Multi-Messenger
Astrophysics (POEMMA), Giant Radio Array for Neurtino Detection (GRAND) and
IceCube-Gen2 have a good chance of observing neutrinos, primarily originating
from charm hadrons, from nearby magnetars. We also show that neutrinos from
nearby magnetar-driven merger novae could be observed in the time interval
between $10^2$ s and $10^3$ s, where the charm hadron contribution is dominant
for neutrino energies above $10^8$ GeV, of relevance to next generation
detectors. We also comment on potential impacts of the charm hadron
contribution to the diffuse neutrino flux.
