Currently this feature is not built-in, but it is very easy to be implemented with our controls suite. You could subscribe to the Layout
event of the parent form, and then check if you have a child form maximized and if so, merge that child's toolstrip with your parent's one.
I have attached a very simple project which demonstrates how to achieve that behavior.
If I could be of further assistance, please do not hesitate to write back.
the Telerik team