To do this without being a helicopter manager, state clearly to them what your goal is. Most employees will work with you if the end result is wfh without a nannycam. Explain you need a way to confirm work product in a way that management can approve of without being too invasive, and keeping on pace with in office work.
In truth, you should not care if they slack off half the day as long as they produce quality work product at a similar rate. Wfh is fewer interruptions and meetings, and means already they will have more "free" time. This is a management principle that is hard to accept, but insisting on the appearance of work is detrimental. Obviously this varies based on the type of work, but yours sounds like a software development situation. "so long as your reports are filed and your queue is cleared every 48 hours, your pace is up to you".
Give them the rope to hang themselves with. Tell them that's what you're doing. Maybe one dev prefers to go easy then crunch for 6 hours on the 2nd of that 2 day cycle, for example.
Ultimately you cannot teach people to be responsible but you get far more positive results from treating people with trust and positivity than with skepticism and monitoring. That makes you less a team and more "the other".
Obviously this advice may not apply in your situation but in general try where you can to apply this in principle.