  The cold dark matter model predicts cuspy dark matter halos. However, it has
been found that, in some low-mass galaxies, cored dark halos provide a better
description of their internal dynamics. Here we give constraints on the dark
halo profile in the Sextans dwarf spheroidal galaxy by studying the longevity
of two cold kinematic substructures detected in this galaxy. We perform N-body
simulations of a stellar clump in the Sextans dwarf galaxy, including a live
dark matter halo and the main stellar component. We find that, if the dark halo
is cuspy, stellar clumps orbiting with semi-major axis ~400 pc are disrupted in
~5 Gyr, even if the clump is initially as compact stellar cluster with a radius
of r_c=5 pc. Stellar clusters in an initial orbit with semi-major axis \leq 250
pc may survive to dissolution but their orbits decay towards the center by
dynamical friction. In contrast, the stellar clumps can persist for a Hubble
time within a cored dark matter halo, even if the initial clump's radius is as
extended as r_c=80 pc. We also study the evolution of the clump in the MONDian
context. In this scenario, we find that even an extended stellar clump with
radius r_c=80 pc survives for a Hubble time, but an unrealistic value for the
stellar mass-to-light ratio of 9.2 is needed.
