I think msscamp and ryder gave excellent answers. If you don't have a reason to expect problems, it may be better to leave them where they are; they will likely be more relaxed. If you can check multiple times a day, just check em often and when you see one go down watch her. If the heifer goes down to calve, and nothing happens in ~30 minutes, you may need to step in. You could give the older cow a little more time, but I doubt she'll need it.
One last thing: When the cow/heifer does go down it's been my experience that it's better to give them room. If you rush up to 10 or 20 feet away, the cow may start to stress by your presence. Having said that, if she's down for a long time having trouble, I don't care if she stresses or not, she needs help.
By the way, do you know what type of bull these were bred to?
Good luck.