If they desperately need a wee 30 minutes after a break then yes they should have gone and be able to last the lesson.
As has been said in this thread a few times, children aren't always as in control of their bladders as you are, and they won't necessarily be able to know if they'll need a wee in 30 minutes.
I'm still that way with hunger. I go from "I'm not hungry at all" to "I'm starving!" in no time, and there are no steps in between for me. I have to cook whilst I'm feeling full, knowing that it's been so and so many hours since I last ate, so it'll come soon. Now children don't plan ahead in that way, thinking "I haven't peed for so and so many hours, and I've had this much liquid - even though it feels fine now, it might not in 30 minutes" - Some kids may have that thought, but not nearly all.