Tonni,
When looking at moving from teaching manuscript to cursive, it is more important to look at how your child is doing on manuscript than on a particular age or grade. Each child should have a firm grasp of manuscript before moving on to something new. For us, that means our eldest is not ready to start cursive, and he is nine. Our next is ready, and he is seven.
Based on what you said about where your children are at, I would recommend either separating when they start or waiting (and if you continue to provide handwriting opportunities over the summer, you may even find your other child catches up in time on their own)
El:-)
