Well, yeah, there's just doing it
You could also discuss it with them - maybe your dad first, then both together? If it's obvious the current arrangement's just a bit much for them - they need their quiet time, time to themselves, etc - and also that they love having the kids, then how about letting them have some input & saying how many visits they'd be comfortable with? Maybe 3 times a week is too often, but one half day would be divine :)