Hello donatas.ciuksys,
Welcome to the Apple Support Communities. I understand you're running into some issues using Screen Time to limit app usage.
When they approach the 30 minute limit, does a request show up on any of your devices to approve more time? Do they get an alert that they're approaching the limit and then are able to continue using the app? Or do they not see an alert at all?
If you set a time limit for different apps or categories, does that work as expected? Knowing the answers to these questions would help rule out a few possibilities and get to the bottom of what may be happening.
You might tap Settings > Screen Time > Always Allowed. Are any of the same games listed under the "Allowed Apps" section at the top? If so, try removing those and then test it out. You can find more about the "Always Allowed" settings here: Use parental controls on your child's iPhone, iPad, and iPod touch.
If this doesn't help, or if the apps aren't showing under "Always Allowed," try tapping Settings > Screen Time, and then scroll to the bottom and Turn Off Screen Time on one of the two devices. Then, turn it back on and re-configure the settings to see if this has cleared things up.
Cheers!