  The free field representation for form factors in the sinh-Gordon model and
the sine-Gordon model in the breather sector is modified to describe the form
factors of descendant operators, which are obtained from the exponential ones,
$\e^{\i\alpha\phi}$, by means of the action of the Heisenberg algebra
associated to the field $\phi(x)$. As a check of the validity of the
construction we count the numbers of operators defined by the form factors at
each level in each chiral sector. Another check is related to the so called
reflection relations, which identify in the breather sector the descendants of
the exponential fields $\e^{\i\alpha\phi}$ and $\e^{\i(2\alpha_0-\alpha)\phi}$
for generic values of $\alpha$. We prove the operators defined by the obtained
families of form factors to satisfy such reflection relations. A generalization
of the construction for form factors to the kink sector is also proposed.
