I'm used to working on week-ends myself, and I have no problem dedicating a large part of my time to them if needed, but I don't want to normalize working overtime and give the impression that I expect them to work on week-ends regularly.
I understand that you want to allow them to work on weekends, but not create any pressure to do so.
Explicitly tell them that they will not get any reward by working on weekends and act like that.
Any benefit you would give, would forfeit your attempts to create a healthy work environment - no matter how you communicate it. Therefore do not give them credit for working on weekends during performance reviews. Do not give raises or promotions based on it.
It is hard, because probably they will be able to achieve more than other employees who have other duties/priorities on weekends. Think about this: Will your childless employees perform on the same level if they get a child next year? Will your employees be able to stop working on weekends and willing to lose the advantage, if they notice that it's affecting their health?
You should reward people who are working most effectively, not working the most hours.
Let's assume you get a high priority task in the morning that must be completed until end of business, whom would you assign it? Employee A who doesn't work on weekends but achieves most in short timeframes, or employee B who grinds through work in overtime? The answer seems obvious here, but is more difficult for long-running tasks. Employee A would be able to up the ante in case of an emergency, while employee B is already at his limit. Therefore think about timeboxes when evaluating employees.
Avoid the impression that weekends are normal working days.
You should not support working on weekends by collaborating with employees like you would on other days. For example do not answer messages and do not handle tool-based requests. You may work on your own stuff, but avoid any interaction and communication with your employees. If they are stuck because they need something from you, ignore it until your next working day.
Having said that, I know that under certain circumstances it's a benefit for employees to work on weekends instead of fixed hours from Monday to Friday. I have been there myself. Therefore I would not actively punish them for working on weekends.