Can a man be a deacon in the church if he doesn’t have children?

Answer:

"Deacons must be husbands of only one wife, and good managers of their children and their own households" (I Timothy 3:12).

"Must" lays down a requirement that cannot be ignored. Deacons must be good managers of their children. That requirement cannot be met without having children, either of one's own or adopted. The Greek grammar here means one or more children. See Must elders have more than one child? for a detailed explanation of the grammar involved.