Client has Sub-S Corp, owned 25% each by himself and 3 over 19 children. The Sub-S Corp (4 employees, all owners)will receive management fees from two LLC's (5 employees in one and 8 employees in other) which Sub-S owns 100% each. If the Sub-S installs 401(k) plan, do they have common control to where the other two LLC's have to be covered?

They will at least have to be included in your testing to see if you pass coverage. Since the individuals in the Subchapter S will all be HCEs via ownership, if the employees in the LLCs are not all HCEs you will undoubtedly have to cover some of them.