Once they hold conditional permanent residence, yes, they may work anywhere. Before that, they have no work authorization from the pending EB-5 case, and their employment never counts toward the ten required jobs.
Family labour is lawful after residence and irrelevant to the job count
Hypothetical example: the owner's husband is a welder and their 22-year-old son a machinist, and both expect to work at the plant. The son, being over 21, is not a derivative and needs his own basis to immigrate or work; he cannot be included in the EB-5 case at all. The husband is a derivative and, once admitted as a conditional permanent resident, may work at the plant or anywhere else, but while the petition is pending he has no work authorization from it.
Neither the husband nor any child counts toward the ten full-time jobs, which must be held by qualifying U.S. workers outside the investor's immediate family. A younger child, say 17, would be a derivative and the Child Status Protection Act's calculation would apply to protect her age during processing.
If the family wants to be at the plant before residence is granted, a separate nonimmigrant status such as E-2 for the owner, with its spouse employment authorization, is the usual bridge and should be planned with counsel.