The critical dynamics of relaxational stochastic models with nonconserved n-component order parameter and no coupling to other slow variables ͑"model A"͒ is investigated in film geometries for the cases of periodic and free boundary conditions. The Hamiltonian H governing the stationary equilibrium distribution is taken to be O͑n͒ symmetric and to involve, in the case of free boundary conditions, the boundary terms ͐ B j c j 2 / 2 associated with the two confining surface planes B j , j =1,2, at z = 0 and z = L. Both enhancement variables c j are presumed to be subcritical or critical, so that no long-range surface order can occur above the bulk critical temperature T c,ϱ . A field-theoretic renormalization-group study of the dynamic critical behavior at d =4−⑀ bulk dimensions is presented, with special attention paid to the cases where the classical theories involve zero modes at T c,ϱ . This applies when either both c j take the critical value c sp associated with the special surface transition or else periodic boundary conditions are imposed. Owing to the zero modes, the ⑀ expansion becomes ill-defined at T c,ϱ . Analogously to the static case, the field theory can be reorganized to obtain a well-defined small-⑀ expansion involving half-integer powers of ⑀, modulated by powers of ln ⑀. This is achieved through the construction of an effective ͑d −1͒-dimensional action for the zero-mode component of the order parameter by integrating out its orthogonal component via renormalization-group improved perturbation theory. Explicit results for the scaling functions of temperature-dependent finite-size susceptibilities at temperatures T Ն T c,ϱ and of layer and surface susceptibilities at the bulk critical point are given to orders ⑀ and ⑀ 3/2 , respectively. They show that L dependent shifts of the multicritical special point occur along the temperature and enhancement axes. For the case of periodic boundary conditions, the consistency of the expansions to O͑⑀ 3/2 ͒ with exact large-n results is shown. We also discuss briefly the effects of weak anisotropy, relating theories whose Hamiltonian involves a generalized square gradient term B kl ץ k · ץ l to those with a conventional ٌ͑͒ 2 term.