Then you stipulate that upon offering a job. You don't just assume that this is what will happen based on an employee having children. As a manager, you manage your employees. If they do something unacceptable, you raise it as you would with an office-based employee.
Why is it apparently so difficult to manage people who WFH? Why is this about assuming less productivity instead of simply following company protocol when work isn't completed in a timely manner, just as you would in an office?
There are numerous reasons why employees aren't productive. Perhaps they regularly come in with hangovers, are lazy, etc. As I said before, if the work isn't being done and you know it, WFH is not some magic circle in which workers are protected from the usual company procedures around productivity/meeting protocol, etc.
The reason employers are not completing work on schedule, etc, isn't the point. It's up to you, as manager, to raise this with them, to reiterate what is expected of them, to give warnings and possibly notice according to company policy. It's not your job to make assumptions about how they will manage the job based on their personal circumstances. It is your job to manage issues surrounding productivity, wherever your team member is based. It is you company's job to provide clear protocol, expectations and steps if those expectations are not met.
If the worker is not meeting targets etc, you take action. That might mean insistence on office presence. That might mean having to ultimately let an employee go. But none of this should be any different for home workers. You employ someone to do a job. How they managed to do that with their personal commitments is not your concern. Your concern is that the job is being done, and if it isn't, your job is to do something about it.