In this paper, the variance-constrained finite-horizon state estimation problem is studied for a class of discrete time-varying systems with an event-triggering mechanism. An event-based transmission scheme is employed during the data communication from the sensor to the estimator with hope to reduce the network burden and energy consumption. Furthermore, redundant channels are utilized to enable the data delivery service to be more reliable in a networked environment. Attention is fixed on the design of a time-varying state estimator such that, in the presence of external disturbances and probabilistic packet dropouts, the estimation error variance achieves the prescribed constraint over a finite-horizon. By means of the mathematical induction method, sufficient conditions are put forward to ensure the error variance is bounded within a prescribed upper bound at each sampling instant. A numerical example is provided to illustrate the usefulness of the proposed estimator design scheme.