  We report single crystal X-ray diffraction measurements on
Na$_2$Ti$_{2}Pn_{2}$O ($Pn$ = As, Sb) which reveal a charge superstructure that
appears below the density wave transitions previously observed in bulk data.
From symmetry-constrained structure refinements we establish that the
associated distortion mode can be described by two propagation vectors, ${\bf
q}_{1} = (1/2, 0, l)$ and ${\bf q}_{2} = (0, 1/2, l)$, with $l=0$ (Sb) or $l =
1/2$ (As), and primarily involves in-plane displacements of the Ti atoms
perpendicular to the Ti--O bonds. The results provide direct evidence for
phonon-assisted charge density wave order in Na$_2$Ti$_{2}Pn_{2}$O and identify
a proximate ordered phase that could compete with superconductivity in doped
BaTi$_{2}$Sb$_{2}$O.
