We did the same thing with my daughters family when they were between houses. They paid all the bills while living there and kept the place up, we still came whenever we wanted. We did not charge them rent.
It helped them out and wasn't much of an inconvenience for us, plus the house was being looked after. I'd keep the whole deal off the record.
The downside could be huge if your kids are deadbeats and tear the place up or don't take care of things, don't pay bills, and / or won't vacate when you want to move back in permanent.