  Context. The origin of the large star-to-star variation of the [Eu/Fe] ratios
observed in the extremely metal-poor (at [Fe/H]$\leq-3$) stars of the Galactic
halo is still a matter of debate.\\ Aims. In this paper, we explore this
problem by putting our stochastic chemical evolution model in the hierarchical
clustering framework, with the aim of explaining the observed spread in the
halo.\\ Methods. We compute the chemical enrichment of Eu occurring in the
building blocks that have possibly formed the Galactic halo. In this framework,
the enrichment from neutron star mergers can be influenced by the dynamics of
the binary systems in the gravitational potential of the original host galaxy.
In the least massive systems, the neutron stars can merge outside the host
galaxy and so only a small fraction of newly produced Eu can be retained by the
parent galaxy itself.\\ Results. In the framework of this new scenario, the
accreted merging neutron stars are able to explain the presence of stars with
sub-solar [Eu/Fe] ratios at [Fe/H]$\leq-3$, but only if we assume a delay time
distribution for merging of the neutron stars $\propto t^{-1.5}$. We confirm
the correlation between the dispersion of [Eu/Fe] at a given metallicity and
the fraction of massive stars which give origin to neutron star mergers. The
mixed scenario, where both neutron star mergers and magneto-rotational
supernovae do produce Eu, can explain the observed spread in the Eu abundance
also for a delay time distribution for mergers going either as $\propto t^{-1}$
or $\propto t^{-1.5}$.
