I would start out with an oxer that has a tiny crossrail in front and set the verticle in the back at 18''. Once you both are confident over the oxer by itself, you can add other jumps to the gridline. You could start with a gride that's a crossrail to an oxer, then you could either raise the oxer or make the crossrail into a vertical.. try different combinations in your gridlines but don't raise anything over 18'' until you are sure you both can handle the oxer. When starting out with the oxer, set the back verticle pole so it is close to the crossrail. Ride it just like you would an ordinary verticle, she should figure out the distance herself. As you both become more confident over the oxer, you can move the back pole further from the front crossrail to make a bigger distance.
Every ride, good or bad, teaches you something new.