Let $G=\prod_{i\in I}G_i$; in $G$ consider all normal subgroups $N$ which are of the following form: $$(\prod_{i\
e i_1,\dots,i_n}G_i)\times N_{i_1}\times\cdots N_{i_n}$$ where $\\{i_1,\dots,i_n\\}$ runs through all finite subsets of $I$ and the $N_{i_k}$ is a closed normal subgroup of $G_{i_k}$ such that the quotient $G_{i_k}/N_{i_k}$ is in $\mathcal C$. Then $G/N$ belongs to $\mathcal C$ for every such $N$, the latter follows from that fact that $G/N$ is a subdirect product of the $G_{i_k}/N_{i_k}$ and the assumption that $\mathcal C$ is closed under subgroups and (finite) direct products, and hence is also closed under (finite) subdirect products. The set of all such normal subgroups of $G$ is closed under finite intersection, so forms a directed set and the projective limit running over all such $G/N$ gives $G$.