Help us!!!

Our twins are 3 1/2 and they hold their poop!! Then they hardly eat because they’re constipated, and then they cry cause their tummies hurt! We’ve tried miralax, suppositories, home enema, sitting them on the potty till they go (this can take up to 30 minutes of them sitting there, and sometimes they still don’t), fiber bars, etc. We are exhausted of trying! What else can we do??? Any tips?? When they finally do go after days of holding it, it’s massive and hard as a rock!

Side note: They have a healthy diet with greens and fiber.