Well, be careful with counting....most months are not 28 days long.
So, let's say that she bleeds on March 25th....that means that she'll bleed again between April 22-24. Subtract 14 days, and you should really aim for sex from April 6 - April 12....that's a long time, but I'm sure you kids can have fun...
Also, for an element of intention, maybe you could try meditating together or praying together, if that's your thing...before you perform the act. Not sure if you're into it, but it could also help.